So far, Yunalesca and Evrae. I died like 11 times in Evrae, just to discover that I could Pull Back the airship (I was fighting with Lulu, Wakka and Auron only).

Yunalesca was annoying for me, but other than that... the trick is to cast reflect on party and just slash her to death. Let your characters become zombies, because doing so they can evade Mega Death, and with reflect, her wht magic against you is unnefective.

I didn't fought Nemesis yet, but everyone in the list other than Seymour were easy. Omega/Ultima... hastega, Wakka, Auron, Lulu... that's the key. BFA, have your characters with good stats and he's done.