SPOILERS so please don't read if you had not finished the game!

I've reached this point now and I wonder how it works. The enemies you meet as Harnakon grows stronger and I wondered if it's because I was too slow destroying the portal or because I broke a few of those crystals.

Also, any recommendations on how to defend? I only had one healer so my next try I'll definitely make sure I've all 3 including the other rangers and mage and is there any cleave on the available weapons as well?

I would like to know this as well, since platforming in this game is weird for me a that is basically what the end game is except for Harnakon.

I tried again and ugh, worst part of the game, just a bit worse than Neuron.

The enemies keep getting harder but your summons suck except the healers, I was able to survive longer now that I had 3.

Also, when you play as your main character there's large chance you could be attacked by something and fall all the way down, losing precious time to advance and keep hitting those crystals (if that's what you're even supposed to do).

I think I give up, it's not worth it. If anyone can send me a walkthrough on this one I would be grateful.

Yeah I may just give up as well, it is a great game but this part is way to frustrating to make me to wanna keep playing it, I may just start a new game on a harder difficulty though if I am feeling up to it.

Find a way, as the main character, to go FAST. Speed potions (or the Speed spell) help, as can Ghost Form (although you won't be able to move levers in ghost form). Sleep and Control and similar spells are super useful. Try to wipe out most screens of enemies; it will just generally make life easier. Get to safe points where you can stop and take a breather before you switch over to Harkanon, because you CAN be hit while idling. But you can also regenerate while idling, so just before you change views you can cast a regeneration spell (or drink a potion) so that you come back in to the fight with more health. Or you can start casting a long-charge-time Heal spell and switch over immediately, while your PC is still in the casting animation.

As Harkanon, you need to worry about dealing lots of damage quickly. Archers and Mages on the tower to the left are effective - but don't neglect ground defenses! Most enemies enter the screen facing and moving left, so sticking a couple archers and mages behind them can help split up the pack, as well as murder the catapults that show up later. Ice Blocks will distract enemy shooters and slow down their melee troops. The Death Cloud spell is great for dealing with masses of enemies, since it won't affect your skeletons or armours - but it does affect you, so don't get caught in it. As soon as you've got things in hand at the siege, you MUST switch back to the player character - the siege is just a defensive position, and one that eventually becomes impossible; it's the PC that actually wins the game for you.

Experiment a lot. See what works. The elven village isn't that long, it just feels that way because you keep having to switch back and forth. If you still have the Orb of Rebirth, you might consider a respec if something's not working for you. Stock up on everything - don't run out of arrows or Magic Essences!

The problem with me is that I just keep falling and falling of the platforms because to jump from one of them to another i have to be on the limit, almost on the end of the platform to reach the other. After the harpies it get's almost impossible because the platforms are REALLY distant, and i already fall at that part 3 times. The funny is that i saw some movies on youtube and they aways seem to pass this platforms really easy and don't need to make such effort, it's really strange.
I'm really sorry for the english, i'm brazillian and i can understand perfectly but writing is not my best.

This, this is the problem. It's so easy to fall down and then you lose so much time and the enemies keep getting harder on Harnakon's side.

One thing that should help is to obtain the Ring of Skies, from the Ghost Tower. Once you reach the left side of the bottom level in that area (mind the weapon-stealing ghosts), go into the room left; you'll need to platform across over a set of insta-kill spikes, and the room is full of bats. (Heat Aura helps.) You'll need to break a few barrels to reveal the ring box, but the Ring of Skies will boost your jump distance, enough to make the platforming during the final segments, while not easy, it'll be less frustrating.
As for weapons/skills, Meltroth makes short work out of everything except the Treants/Ents. When faced with many enemies, have Gatchan out, or repeatedly cast Sleep. (If you were diligent in questing, you've got it, if not, find the Spirit Box in the Library and do its quest. Golden essence, 2 per cast. Don't have Gatchan? You can find him behind a door in the halls that takes you to a even-more-retro platforming challenge.)
Once you reach Corvax, back up, make sure Harnakon's counter isn't ticking down, take all the buffs you've got with your best weapon, and get in there. If you have an Arcane Immunity scroll, use it as well, that will make things a lot easier. (Didn't have one myself, so I just offensive buffed and wailed away.) Corvax doesn't have that much health, really, he/she is just a teleport-spamming glass cannon, but the platforms block the mage's bolts. (Yeah, I think Corvax is a she, though, due to the fact that that mage wears pink.)

On defense, Ice Blocks are usually a good distraction for most waves, and the Skeleton Warriors, though the latter die easily. Red and Green Armors are more durable, but you'll want to save Harkanon's green essence to summon Goblin Healers first. (100 green essence! Geez!) Repeated casting of Cloud of Death will quickly kill Elven Mages and Archers (one shot!), Warriors (two shots) The Treants/Ents are a lot more difficult, but with Skeleton Warriors running interference, your Armors should be able to handle them. Skeleton Archers and Mages should be perched on the ledges to provide ranged support against everything; and the Gryphons/Harpies, whichever you want to call them, die quickly to Cloud of Death, cast from a ledge.
Under no circumstances should you touch your own CoD, though. It hurts, a lot.

This bit had me setting the game aside for a week to cool down after a dozen tries... well, that and I wanted to see if real time passing counted towards the 5-day timer. As in, I wanted the bad guy to lose. After what he did to Zera I don't care if he's a decent-sounding fella largely just minding his own business, I want him to lose. Unfortunately the timer doesn't work like that. Maybe if I leave the game on for 5 days....

Here's how I eventually beat the platforming bit, hope it helps.

I had a lvl 17 magically weak character - maxed (or almost maxed) swords, maces, axes, bows, armor, and constitution. Just enough healing magic to have the anti-poison spell (Medusa-Knight was my 'That One Boss', the giant brain was actually easy once I had my Ah-Ha moment). The basic lvl 1 spells in each category. The floating-ice-shield spell. I hadn't used the rebirth orb and ended up not needing to.

With golden armor and Excaliber I ran through the lower levels. I thought about invisibility potions or ghost phasing or speed potions but none ended up being used. The ice pet was out and constantly energized with those green scrolls from the shopkeep. If an enemy wasn't directly in my way I usually didn't bother to kill them. Easy-to-reach archers, mages, and the crystals were exceptions. The bottom two lays or so are real easy once I had my running pattern down - there isn't much incoming fire and the ice effect slows it down even further. It took a few sessions but I figured out when I was about to be attacked in the army-vs-army mode and where its safe to hide (either because platforms block incoming attacks or because the screen is easy to clear of all the enemies)

During the invasion sequences I just spammed 4-5 cloud-of-death spells 2 seconds before the next wave and hid at the top of the ladder, half above, half below the platform, where he's hard to hit. Once the trees popped out, 4-5 clouds, a bunch of red knights, and the fire-breath make quick work of them. If you get to the flying enemies you've taken too long.

On the first screen where the macemen show up I switched to a weapon that hits multiple targets - my weapon of choice here was the high-end unique hammer. Try to clobber them all while they're together. If you get pincered they're pretty slow, just swing left/right/left/right/etc until they're dead.

Up the ladder from there are the tree ents. I didn't fight them. I climbed as high as I could, inched off the ledge, landed at the switch, threw it (you can flip the switch faster than the archer can shoot you point blank), and worked my way, jumping over a lot of ents, to the next screen. I'd leave this one in such a state that I could stand at the right edge somewhere and be protected from incoming - there's usually a new wave around this point, but I've left the last two sections full of enemies and the next one takes a minute.

Two flying firebreathers in the next screen. I'd never used my scrolls of Midas Transmutation, and they're worthless against the upcoming boss fight - this is the place to use them. On my final run I actually Transmuted one and sleep-spelled the other. Taking out the healer while they're asleep and only fighting one at a time makes it much simpler.

Before the boss fight I switched to the floating skull - his instant beam attack is more reliable than any other attack I could offer. I went up, got a good vantage point, and ice-shielded both sides of me. If he got close enough to whack from behind the shields I did, but skully did most of the work.

Falling down twice I still managed to finish it with the first wave of ents topping out the difficulty curve at the defensive point. Don't waste time and you can afford a few mistakes.

If you're looking for a way to cheese the endgame, you need two - and only two - spells to clear the various screens in the elven village with relative ease.
Under Mental Magic - Control. This spell instantly turns an enemy in to an ally and can take care of every enemy that isn't a crystal or a boss. It counts as an instant kill, so if you Control every enemy on the screens that you have to kill all enemies, it will open things up.
Under Protection Magic - Reflect Projectiles (not certain of name, but it's top-level). This is handy when dealing with archers and fire-mages, but it's absolutely stellar against the boss. When she gets tagged with her own projectiles, she gets summarily stunned, making her easy pickings for a melee fighter.

The Ring of Skies is almost necessary to make a lot of the tricky jumps. Healing spells or potions are handy.

Oh, and for a quick wrap up of the last screen, Cone of Cold will make quick work of both of the big crystals (although you still have to shatter them with something afterwards).

The downside with this is that you have to save a bunch of skill points for some late-game power sets that don't do you much good outside of the last areas.

How I beat it:

Harnakon: deadly cloud wipes out pretty everything except trees. In the first 4-5 waves (up to trees) when the countdown
reaches 3-4 cast 2-3 deadly clouds, you don't even need to stay there watching. During this time you can make a good part of the eleven forest with Daniel. Against gryphons also deadly cloud or frostbolt (better spare fire magic for trees). Forget the summons, except to keep enemies busy while you cast deadly clouds or fire breath. I never went in melee with Harnakon, it is suicidal, only spells. Defensive spells: cold blast against gryphons (mages shouldn't even cast after deadly clouds), improved healing. I never summoned healers myself. Also, trees can't climb the ladder, so you can even avoid fighting them in some occasions.

Daniel: used the acid bow a lot for casters and archers, excalibur for melee elves or some archers, axes for trees. Spells: sleep, regeneration, fire spirits, some fireballs. Against trees the pet that can stun with rays is particularly useful. Keep ready one arcane immunity scroll for the last mage, and keep the ring of skies equipped (necessary). After you killed healers in one screen, if the countdown starts and you have to shift soon to Harnakon, cast 2-3 fire spirits and go to a safe place. You can use speed potion (I had over 10) but I've mostly used them at the beginning.

Here there are some more tips

http://www.unepicgame.com/phpBB3_en/viewtopic.php?f=5&t=1263&p=7010&hilit=ending#p7066

Why did anyone think this section was a good idea...?

This section resulted in a few rage quits followed a rage uninstall. The main problem is that it's long and if you screw up, you have to do it all over again. Reminds me of the old nintendo platformers where the level was long but fair up until the end where they'd have some crazy distant jump you needed to make pixel-perfect, plus an enemy that would fly out while at the top of your jump giving you a window of 0.1 second of error. NOT fun.

The harnakon battle is a mess. You'll have a dozen or more guys all overlapping and you can't distinguish friend from foe. I'm breathing fire into the mix and I have no idea if it's harming my allies or not. The only strategy I could get to work decently was to get harnakon on the far right-side of the screen, then beat the elven mages to death from behind so they couldn't heal anyone. I took very little damage when doing this.

I didn't know you could sleep the dragon bats or whatever those things were. When my character got to them in the elf village, I couldn't seem to do anything. Any projectile spell I'd use would miss because it was too slow and then I'd just get pelted by fireballs until I could retreat and try again.

But come on.."Hey! The end is really Cool!" ;)

It would be nice if there was a file that had the jump height as a modifiable attribute... that way I could increase it JUST for the time I was "playing" that awful section. And/or to get that Ring of the Skies.