Sorry to nitpick, but you listed two different armor classes. Do you mean you have two different fighters?

Good armor class definitely helps, but it is never a guarantee you won't be hit. Basically, save a lot. Some experience loss in this game, as well as Pool of Radiance (and many other gold box to a lesser extent) is probably inevitable. Again, turning with your cleric in this game may be your best option in this situation. (Note that turning works very differently---and much more annoyingly---in Pool of Radiance; you may want to avoid using turning there with a cleric under 6th level).

Worth noting that every attack will have at least a 5% chance to hit (and at most a 95% chance), so there's no way to eliminate the possibility of being hit.

If things are implemented properly, having your fighters cast Mirror Image might help them avoid getting hit and hence level drained. Of course, this requires that your fighters somehow get access to the Mirror Image spell, perhaps by multi-classing. (I believe thins may not work in Baldur's Gate 2, as there I believe Mirror Image is implemented as negating damage effects rather than attacks, so it will negate the damage but not the level drain.)

Dtgreene is right. Mirror image is one possibility. I forgot about that. As she noted, it only affects the caster, so you would need multi-class here. (Dual classing, as far as I can tell, is not available in the Krynn games anyway. Please tell me if I am wrong.) One thing to note: according to Stephen Lee, in a couple of the early games, such as Pool of Radiance and Curse of the Azure Bonds, mirror image is somewhat bugged, so it does not work as well as it should. Note that are no level-draining undead in Curse; in fact, there are almost no undead at all in that game.)

Actually, in Baldur's Gate, I"ve found mirror image one of my staple spells. She is right, though, about how it works there, which is probably closer to its tabletop version.

I have a knight who has -9 due to the Solamnic Plate and a +3 ring, the one with -6 is a fighter.

The AD&D spell creates duplicate images of the caster which disappear when hit, the same as in these Gold Box games.

I don't think the BG version of Mirror Image is particularly faithful to the tabletop version. In particular, in BG:
* If there is an image, the image is always hit; there's no chance of the attack actually hitting the actual target.
* BG Mirror Image will block area spells like Fireball, whereas I'm pretty sure that, in tabletop, the spell will ignore the Mirror Images, with all the images appearing to have taken damage.
* Some attacks in BG cause multiple instances of damage; Melf's Minute Meteors from BG2 is a good example. IIRC, each hit does 3 instances of damage; the rolled 1d4, the + 3 points, and then the 3 points of fire damage. Each of those instances will dispel a mirror image, even though there's only one attack roll for this. In tabletop, I'd expect each meteor to hit just one image and remove it, or to by chance hit the real one with all 3 damage effects.
* Some attacks, like the level draining attacks of some BG2 enemies, do other things that Mirror Image does not protect against. So, a vampire's attack's damage will be negated by the image, but the character will still be level drained. In tabletop, I would expect the level drain to be blocked along with the damage because the attack didn't actually touch the character.

(Note that this info is based on the classic edition, with no mods; it's possible that the EE or mods may chance some of these details.)

They did make some changes. For example, spells that stack in classic don't stack in EE. (Armor of Faith, for example.) You can't re-cast Project Image until the duration of the initial cast would have run out. (That was done to fix the exploit of using chain contingency to spawn multiple images, but it also hurts what I've always seen as a legitimate use of the spell.) Also, apparently a later update made it so that you can't use Resurrection or Mass Raise Dead to heal living characters, a change I strongly disagree with (and which was presented as a fix for a bug rather than a balance change).

The fighter/cleric now gets level 9 in both - how is the half +1 HP for cleric calculated? Rounded down to zero, i.e. never getting HP from cleric level-ups again? And when getting to fighter level 10+, every time rounding down from +3 * 50% = 1 and missing out 0.5 here too for the rest of their advancement?

Actually, maybe it changes, but so far I could have made one fighter/cleric a bare fighter as he rarely casts at all (is instead an albeit weaker melee char beside the knight and the fighter). The other fighter/cleric cast sometimes and mostly uses the bow. She would suffice for buffing as sole cleric (maybe later I will make more use of cleric spells).

Do I overlook something or do white mages have no protection spells? Both mirror image and ironskin are red only in this game...

My understanding is that you always get at least 1 HP when you level up, even if your HP gain would otherwise round to 0.

Looking at the spell list, it looks like white mages get (Minor) Globe of Invulnerability, as well as Mind Blank. The former means that you can, for example, have a fighter/white mage charge into the enemies with that spell active, while another mage casts Fireball on the enemies, and the fighter/white mage will not be affected, at all. Of course, it's also useful against enemy spellcasters, at least when they use lower level spells like Fireball.

(Can enemies run out of spells in this game?)

I know that in the FR Gold Box games enemy casters do have limited spells, so I would assume the same applies here.

Just trained the fighter/cleric to level 9 for both. According to the tables, this means 1 HP for the cleric and 1D10 HP for the fighter, so 1 + 5 = 6 for this multiclass level-up.

He just got 12.

Then, the mage leveling from 10 to 11 got +5 HP.

Won't complain, but was surprised.

It is tedious but possible to keep saving and loading in gold box until your characters get maximum (or close) hit points on each level up. The quickest way to load a game from a training hall in most gold box is (from what I hear) to remove all characters from your party, which immediately takes you to the game's initial loading menu. (A load command was not included on the adventure screen until The Dark Queen of Krynn.) Of course, this only applies until you reach the level where characters get a flat amount of hit points each level up.

I completed the High Cleric Tower and Durfey is still with me. But his charging in battle (if he can not be controlled by me) is annoying, so I want to get rid of him by throwing him out at a training place.

But as he plays a role later, would I break the game this way?

I read that after completing the Cekos side quest, a magic shop is available there. Does it really sell only darts and arrows? If so, are there no potions and stuff sold in this game (I read this might be the only magic shop)?

No, it won't break the game to not have Durfey with you, though you will miss out on some plot things. As far as I can recall, it never breaks the game to have or not have an NPC with you in gold box. Yes, NPCs charging into combat can be a pain. That is why it is good to have a high charisma in D&D gold box, as that will usually let you control your NPCs in combat, except in the first two, Pool of Radiance and Curse of the Azure Bonds, where the computer always controls NPCs. (In the Buck Rogers games, there is a specific Leadership skill that does the same thing.) In your case (and in any case for a party without high charisma), I would recommend giving Durfey a bow and plenty of arrows and equipping them. That should keep him in the rear lines in combat. (It's possible that won't work, but I think it will.) Just one more thing to remember: NPCs are not allowed to trade items to other characters while they are conscious.

It's been a while since I played, but I don't recall anything about a magic shop in Cekos, and I can't find anything in the clue book about it. The Death Knights of Krynn clue book does tend to skimp on details sometimes, so that doesn't mean for sure it isn't there. Sorry I don't know anything about this.