They're all decent choices.

Personally, I've never taken my warmaiden or fellhound past 50, but there's a good argument in favor of maxxing them out.

Orn can solo dragon and ice golem 20 in a good regen set, so I would probably do him, then you can grind those dungeons and bring chickens to graze.

IIrc the stats he needs are roughly 70k hp, 3k def, 220 spd and 300 res

Without context the easy answer is "nobody". Full roster. Go.

@kramaswamy.kr Wait for better options then? Or just pick one?

He's saying show us everyone you have before he gives advice which to work on next