Make good use of the Time and Combos, it is very important that you try and rack up a high combo streak - such as 25-40. Don't use the Combo Time till the Axe man arrives, since that's when loads of Majin will spawn.
At the beginning till the Axe man spawns, you should try and finish enemies with melee to get an extra 5 seconds added to the time, Wesker's Cobra strike can 1 hit everything. All you need is a Flash Grenade or a head-shot stunned enemy.
For A Ranking on single player, I think it is 60k points. On Co-op, it may be around 80k+.