Taking on your advice for a team setup first, I brought out those exact heroes:
Gullibursti / Tiburtus / Rigard / Domitia / Proteus
w/ Super Mana Potions, Mana Potions, Time Stops, and a Miracle Scroll.
For the most part, I managed to survive both Ymir and Surtur after clearing the first phase, but the moment I got to Nidhogg and reached halfway, two heroes (Gullibursti and Domitia) died. I thought I’d press on until at least 1 more died before I used a miracle scroll, but Nidhogg got charged up and (World Tree at x2.0), his special skill wiped out the rest of the team.
Did the forbidden action of using gems to continue because I was frustrated and had already taken advantage of the reduced WE and battle items to let this go to waste. Managed to kill him with maybe only Domitia (for the second time) dying.
Down the line, I’ll retry it again with maybe higher-tier heroes leveled up. Maybe with suggested Vivica or with my Grazul again after I max her. (she seemed to help negate the debuffs whenever I couldn’t avoid the special skills, like with Nidhogg.)