The one on the ship, I built up wakka and tidus till they got overkills every single time. Then, when I went to go fight Evrae, I just kept the ship where it was and put Hastega on Rikku, Wakka, and Tidus.
I used Rikku as the healer, and Tidus and Wakka as the Attackers. Everytime I pulled out, I kept getting hit with that spray that thing uses to turn everybody into stone. But yeah, I just used the overdrives and kept hitting hard with Wakka's attacks. . .after about two or three rounds-Evrae died pretty quick.
If you're talking about Evrae the zombie-just throw phoenix downs on it. . .I didn't even bother with the locks, because afterwards they all unlock. Afterwards, make sure to look around for chests. . .they one contains Rematch, and I forgot what the other had.