It is impossible for me to kill Gyendal in the final battle after I brought Mel to the Orb of Life
Q : How do I get him killed?
 
Stella has the All powerfull "Mayor heal Party" spell, that just cures ALL the damage on him I inflict.
 
I just cant get stella to be stunned, and killing her is impossible, her resistance is absolute (1 damage per hit)
 
Q : How do I get her stunned, so she wont heal Gyendal again?
 
Also Gyendell has one Very powerfull attack that whipes away 95% of all my health of all my chars  "Lightening storm" or something like it.
 
Q: what defensive amulet should I wear before confronting Gyendal?
 
Lydia has learned 16/20 spells so far :
*5 storm spells
*5 fire spells
*5 ancient spels
*1 fairy spell
 
I have all the best weapons, and items in the game.
All my characters are level 35-37
 
-In my inventory is a healthy supply containing :
Healing & Restoring :
 
*10 restore all mana for 1 player potions
*10 restore all health and cure illness for 1 player potions
*3 restore mana for all players potion
*7 restore all mana AND all health for all players potion
*10 of all items that  take away a negative effect
*20 Revive player leaves
 
Attack items
*2 attack rings
*5 of all the types of scroll
 
Misc items
*16 +1level eggs
*1 of every gem that fits in exalibur
*4 of every type of amulet in the game
 
Please help! I have tried the final battle so often! like 20 times now!
I cannot get the evil lord to die!

Done IT!
I could not help keep trying, and after ALL my spells faitled, all my items failed I though what the heck last try..
 
*Used all 16 leveling eggs
*Used all the +100 mana roots on Lydia
*Used all the +5 strenght items on my Orc
 
Found a fairy spell did kill off 9000 per round, once cast!
also my raw power now raised to a nice 11000 per round
 
Stella still only got +5000 health every time she casted heal party.
A few rounds later the evil lord was dead:):)
 
now just wait for Aveyond 3.3:)

Hi! I just wanted to add my experience so if anyone else needs help, they would find it here.
My party had: Mel, Edward, Te'ijal, Galahad. First round, i made Mel use a spider egg, all the rest use physical attacks. The spider egg is sooo  powerful!! It causes a damage of 10000 for Gyendal. So technically, my party would remove the HP, Stella gives him & the spider egg does the rest. During the other rounds you just have to stay alive till the spider eggs finally does it. I used Te'ijal's draining power & made Mel use the red potion (that returns 100% HP to all the party members) each time Gyendal cast the "lighting grid". I had 7 of that potion. I used this technique twice & it worked both times. Good luck =)

Yes, Heero is definitely right about those spider eggs! They inflict 10000 hp damage per round, which Stella can't possibly make up for! I thought another strategy and some hints might help as well:

--Before you fight, make sure you have plenty of Tinctura Hypericum(restores Health) and Aquifolium Extora(restores Mana).
--Your party should be around level 40 or so in order to fight Gyendal. Make sure you use all level eggs, mugworts, and mandrakes you find!
--Also, since there're some of you who haven't played Lord of Twilight before Gates of Night(and therefore wouldn't have the spider eggs) Lydia's Succubus spell works similarly, causing initially no damage, but mega amounts of damage with each turn.
--A good item to have is the Medusa Sword, one of the items you can get from defeating Medusa. If you save just after fighting her and just before looting the corpse, you can keep reloading and relooting until you get it. I've never had the patience to get it, but others have said it will turn both Gyendal and Stella to stone, making the battle much shorter.
--A final hint: make sure Lydia has all the Fairy spells! Her Siren spell is amazing on pretty much all the enemies!

That said, my strategy was to have Mel, Ed, Te'ijal, and Lydia in my party.
Have Mel use Trip every round. I know some might think her special skills are too weak for bosses, but it works surprisingly well on Gyendal. Using Trip, Gyendal was virtually never able to attack.
Have Ed attack physically, with Excalibur Sun(obtained by attaching a Diamond to Excalibur and dipping the weapon in the Fountain in Aveyond)
Have Te'ijal use the spider eggs(if you have them) and also make sure you found Crusher in the Memory Caverns. It's the best bow in the game for her.
Have Lydia first use Succubus, then have her use either Siren or Mortmal's Swarm.

If you need to heal anyone or restore Lydia's mana, have Te'ijal do it. While she's one of the strongest members of the team, Ed's Excalibur Sun does far more damage to Gyendal.

Hope this helps anyone in the future!

Thanks a lot for the tips! I also managed to win the battle now with the help of spider eggs and succubus... (didn't even have the siren spell yet) In addition to the health restoring items, I also found easter eggs an enormous help, when I ran out of the tinctura's. :)