I don't think there are any fiends I would class as 'hard', but there are plenty I would class as 'annoying'. For example, Great Malboro - Bad Breath, Dark Yojimbo - need to defeat him five times, Demonlith - petrifys my team when I'm not looking, Queen Coeurl - Blaster KO's one person instantly...I could go on, but I think that's enough for now.