How you kill Ktahu

[klejf]

i cant kill him as well, maybe something is wrong with my army, that is:

15 rdrags
70 executioners
110-120 demonologists
160 knights
140-50 horsemen

i have around 40% physical res on any of them, 35 defense as mage (lvl 52, hard dif) and 25 attack, also got the +20%ktahu dmg blade

my problem is that i hit him for ~2k max, it was acceptable vs gremlion, not that acceptable here

also got 15 archdemons and 10 b drags in reserve

any suggestions?