What are the most dangerous unit combinations in Dungeon?

You don't necessarily have to attack. You can just run up to the stack without attacking (and with Alt-Knights, you can even attack without retaliation) and use the "Bite" ability as much as possible, which is like a full attack but with minimal damage. Neutrals die very quickly.

As far as I know, many players leveled up Sorghall this way - a stack of Knights and 6 stacks of Furies. Neutrals were easily defeated.

And I think the most dangerous combination is Red Dragons with the Vampirism spell. Their 1.5x damage from Devastation allows them to recover a lot of HP, considering the Warlock's decent Spell Power...