Battle game!!??

Okay I've unlocked battle game for Resident evil code veronica X and I have Claire, altrenate Claire, Steve, Chris and wesker, I'm struggling to even get to the boss with wesker does anyone have any tips, oh and i always get an e ranking at the end of battle game, anyone know what you are judged on and what affects our score so i can improve?

I remember that you can get a magnum with Wesker, but I don't really remember how to do it. It's in the Casino though, I know that much.