I usually use Sigillaria as packed debuff. Then, the broken combo of Adlay+Zinnia+Nidhogg (previously Corbett) delete everything from existance. I add Spring Star (Parallel Academy) for extra guts, HP drain and damages. Nothing in the game can handle this much damages.
I think Adlay is the key here. But I was too overpowered for Nidhogg and didn't really pay attention to it. Same for Midgardsormr. Hraesvelgr is another story. I brought the big guns and beat it in one try but it was just pure luck. This thing is stupidly strong.