1. Go on to the boat. One boat has a blitzball. Press x next to it and get rid of the memories to get the shot.
2. Gillionairres is not a must have. Just beat tons of mimics in omega ruins. (Steal from the treasure chest next to the lizard and they sometimes appear.)
3. Just avoid Dark Yojiimbo and fight him later. All monsters can be caught without meeting him and by staying at the front. All apart from Tonberry, so when u caught others go right up to the end where the fayth is and capture him. But if you exit ull fight dark Yojimbo, so instead do on the teleporter and teleport back to the entrance avoiding Dark Yojimbo.

for more detail check some detailes FAQs.