Fight a week person with pieces and level them up or you can fight someone strong but don't use the pieces and at the end of the battle all the surviving pieces will get exp do it a couple time to level the exodia pieces.

Monster have to destroying symbol

Attacking a symbol always scores 10 exp points per attack, and usually 100 for destroying it. As for beating Kaiba, I recommend surrendering and retrying until you start first so you see what monsters he has, then choosing monsters with type advantages against him (e.g. he has 2 light, send 2 thunder, he has 2 earth, send 2 dark etc.)