To those who aren't getting the impossible badge, I tried most of the combinations suggested here, and the one I was successful with was: 3 rogues and 1 cleric, keeping the 3 rogues at the front and the cleric at the middle back. Equip your rogues with health regeneration and your cleric with power regeneration. Give the rogues 2 points str and 1 point end every level, and your cleric all int. The best way to play is to make all your rogues use the 'create opening attack' every time, and keep your cleric's mana reserved for healing and revive. Btw, don't equip your cleric with heavy equip just cause he can, but always look for the ones with best power reg! Worked for me, hope it helps!
 "Monsters' Den"){kind=icon}
 "The Necronomicon"){kind=icon}
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 "The Last Stand"){kind=icon}