if you are waiting for 8-6 and are stuck heres how i did it eventually:
equip: 2 dimension axe 2 sunshield 1 ragnarok (could have been another dimension axe) 1 meteor shield. you should have 3500+ hp 4500+ atk and 3500+def
use steel armored popo. in the beginning go right and iceblaster the black dragon, summon twin diablos then ice blast the dragon again. you should be able to do this 2 times before the dragon reaches your end of the map. If you killed him already great, if not summon Pegasi and iceblast the dragon to doom. now you have to defend your way out. AI should summon pegasus knights at tons now. try getting a second twin diablos out and summon pegasi yourself. use heal if youre below 40% health and keep your units alive. diablos wont make it through hostile pegasi but they will tank damage so your own pegasi can take out theirs.
Eventually you will have more pegasi than enemy and you will begin moving forward. use heal to keep pegasi alive and safe your mana if AI takes another unit. Else summon twin diablos again.
Your pegasis will die on their way to the enemy hero but diablos will survive and kill her. Use rage once they reached her.
its hard and takes time but it will do.
If oyu have them you can also use 1x Elixier and 1x khronos shield. probably best with 3x dimension axe and 1x meteor shield. having 3500+ Def is way to go though as you will take quite some hits from dragon and pegasi early on.