Since my last message, killed both Zilgadis and K'thalu with a single stack of red dragons as my army.
For Zilgadis:
I couldn't wipe out all the tower spam, so I concentrated on killing Friendly Gremlins towers ASAP to avoid to end overwhelmed by ennemy troops, letting the Evil Gremlins live on purpose and tanking their spell damage.
I had 15 Red Dragons (level 53 mage + dragonix) and they survived during 117 rounds to the spell spam of 4 Evil Gremlins towers + Zilgadis himself.
That's long, but as soon you know perfectly what you are doing, that wasn't so hard. The only difficulty is to not get your starting position taken when you move to kill summoning towers (it forbids you to cast Turn Back Time)... or when those Emerald Dragons grabs your stack and force it to move.
For K'thalu:
My bane was lizardman shamans and their dreaded "blood mark" ability, that double damage recieved by my red dragons... allowing K'thalu to destroy 9 red dragons at once
I had to do multiple tries to find how kill them fast enough to not let them use the mark.
The Death Star spell helped a lot. I kept carefully any spare mana I had between Turn Back Time castings to be able to cast this spell when 2 shaman pack were summoned at once.
Was a bit tricky, but after 141 rounds of "get out of there, ******* shamans!" shouts, I won.
I'm currently struggling A LOT with Baal.
Archdemons are the big problem: if they halve my stack (7 dragons killed in one blow), then Baal's flying sword (5 kills each time once Gilbert's units are dead) and remaining ennemy troops just finish them off easily.
Also, archdemons are much more resiliant than lizardmen shamans.... so wipe them out is harder.
AAANNND demons are also fire-resistant, so Fire Flood isn't doing much damage.
I need to change the type of creatures I use in my single task for something with more hp.
I ran some tests, and it seems that I should be able to kill him with Ancient Ents or Brontors without much problems... once I understand the arena collapse pattern that wiped me two times already.