How are you supposed to get the rank of A? I want to unlock new characters but I can't seem to get past B.

Its best to try co-op, I think it is 60,000 for A Solo and 80,000 A Duo, but I could be wrong.

It's 40k for solo and 80k for duo.  BTW there is a S and SS ranking as well.  Some of the levels are really hard, and a few I got A on my first try.

I'm trying to get A on Mines to unlock Wesker, but the bug thing really messes me up every time.

Prison is the only one I had to do more than once to get over an A, you just need a good partner.

run from the roach, don't even try to fight him without a partner, use sheva and aim for combos(shoot them in the leg if they are rushing at you, the arm if they are winding up for a swing and the head if all else fails, when they stagger punch them in the face and stomp them on the ground afterwards).  You'll get the hang of it after awhile.  Try and hit all of the time things at the start, there's nothing more annoying then having less than 30 seconds on the clock and trying to get a time thing that is stuck behind an enemy that will kill you if he touches you.

"We are talking about a few tips here. A solo is 40,000 duo is 80,000. "