I have fought this guy several times, trying several different strategies in Classic Mode, and he just won't die. He constantly heals. Is there a key to killing him?

@LordDragonOCT in the immortal words of Arthur (Monty Python Holy Grail) Run away Run away.

Joking apart that is how I beat him on veteran by running away.
I left the mage, cleric and thief at the bottom other end of the map. The place with the gap between to rock areas that cut away to the left and right at their bottom. Cleric and Necromancer in right cut away and Thief (using bow) in the other.

The two fighters went and engaged him when one got to about 2/3 health and one of his copies was dead they ran away. He chases and the fighters train him from one end of the map to the other all the while he is firing off spells most of which don’t hit.

Did two full circuits before the fighters ran down the gap. At the same time the Necromancer summoned some pets into the gap. The pets engage the train, when the train gets into the gap the fighters have healed up.

The Necromancer continues to summon pets into the gap, between pets and the fighters he and his copy never make it into the gap. The fighters beat him and his copy down the cleric heals when needed.

The reason I did it this way was that he seems to target the caster and thief with damage spells and two hits is normally enough to take them out. If he or his copy do manage to target one of them ducking back into the cut away puts them out of range of a second hit. Any pets he manages to summon behind the fighters between them the Necromancer, Thief and Cleric can take them out.

That was the tactic I used for this party it might not work for yours.
Helgenhar Fighter
MC Fighter/Shaman/Winter Mage
Siracca Cleric
Hamlin Thief
Jade Necromancer

But what ever party you have if they can summon pets then do it. Throwing pets at him is always a good tactic as he seems too always engage them before targeting your party.

For me the hardest battle is his skull form. Really nasty bugger who spams very damaging spells.

I found using Gale Shroud to be extremely helpful. You need at least 4 of them, so I have Bjalla and Sea Hag both having 2, which gives enough time to wear him down while his spells are ineffective inside the shroud.

My party:

Main char (ranger, for this battle using melee)
Bjalla
Jade
Sea Hag
Helgenhar

Those who have more fighters in the party might have easier time since you basically wear him down with melee attacks. But I went with mostly mages. Summoning all kind of support like ghouls helps too.

@shmerl I found the skull easier, I just used 4 of my party left Hamlin out of the fight at other end of the map. The fighters engage him, the cleric stands a little further back and the necromancer stands a little further back than the cleric out of range of his spells.

When he shields up and starts firing his fire spell the fighters disengage while the necromancer fires pets in his direction to stop him advancing. When his fire spell stops fighters reengage. Rince and repeat when his fire spell goes off.

The second or third time he uses his fire spell he has no shield up so is also damaging himself. Then my MC fires their pets at him, and necromancer drops her big fire spell on him the sets a big area aflame.

When he reaches the edge of the flaming area his fire spell has finished. He can’t get out of the fire zone as my fighters standing at edge (outside the fire zone) won’t let him pass. He dies quickly from fire damage.

This time round (still Veteran level) I think I can do all of castle Alastor’s fights on auto. My character this time is a Templar who I soled to level 20. At 20 I formed my party with Helgenhar instead of Hamlin and not once have I had to get involved in a fight, I have just let my party do the fights on their own.

As they are all level 20 now I can’s see any reason why I would need to get involved. Hell, I recon I could solo it on my Templar. I am going to save before the castle and try it with just the templar after I have finished it. ;)

I didn't get to lvl 20, most chars are like 17 or so, so that fight is pretty tough.

SPOILER:

The ending is interesting. I was expecting Isla to be somehow connected to Zoria (or may be being an avatar of Zoria or something), but she ended up being called the Guardian by the Pendulum. Interestingly, you have an option to romance her.

In my playthrough I accepted Zornilsa's help and freed Zoria, but didn't side with Zornilsa directly. Feels like an interesting set up for the sequel.

For my next play through (probably after the DLC) I am going to try the greed way. So side with the secret benefactor and accept Zornilsa's help. But this time on Courier level.

I feel like there is some kind of path of getting a better story direction where both sisters are reunited in some positive way, since Zoria mentioned that Zornilsa was poisoned. You can also notice she herself has some kind of split personality when she changes her voice, more friendly one telling the MC that she won't forget helping her.

All that assuming they plan to continue the story :)

I hope they do make a sequel I have really enjoyed this game certainly worth the price. I have manage to get 150 hours in and not one crash. A couple of bugs early on but nothing that prevented me from playing through them to completing a zone or quest. :)

Yeah, I liked that it's somewhat less imposing in scope than let's say Pathfinder games (which are very good, but much bigger), but at the same time the story is enjoyable. Good balance! I hope too they'll continue.

Special kudos to game writers - they did a very good job. And someone had fun writing in-game books, they are like little horror fairy tales, really well done.

Just to let you know I did the castle on auto. The only interaction on my part was in the Lich Argos Agelas fight had to move my archer and mage out of aoe damage are twice. Also last fight where I had to move group to each of the chains. :)

Have to say the party AI is pretty good.