Underworld vortex has been the easiest of the three so far. The only thing about him is that he's crit resitant and stealths if you don't kill him fast enough. Just have your strongest light units and he'll go down in two turns, even without Maxwell.
In my experience Ultor's dungeon (Machine of Destruction) was the easiest, mainly because Cyclaw didn't really have any outstanding mechanics. That, and I had Ultor tanking for me which actually helped in the dungeon.
Turbo Cyclaw was tough because of the shield, and I just got messed up due to the stealth mechanic of the Turbo Netherhound. It's funny how it almost feels like a bug when you can't do anything, lol.
Yeah, maybe it's just the fact I don't have strong enough units to knock him out quickly enough. I could try burst bombing my Ultor to max BB rank and try the tank strat, I guess.
I do have a few Maxwell friends, so that could be an option too.
edit: Managed the tank strat well enough, Ultor is such a silly unit and has good synergy with Lilly Matah. Funnily enough I could execute Ultor's BB when doggie was stealthed, which meant I took basically no damage from his stealth move since he was pounding on Ultor - unless he decided to use his aoe, which had me using a few rezzes. Got a cap to boot, if I ever accidentally summon Hadaron