Use the RPG with infinite ammo enabled, then just rush through the level. Kill every enemy you encounter. Because you are using the RPG, enemies are killed easily from direct hits and splash damage. Your accuracy will be rather high. Because you are killing all the enemies your kill count will be high. Because you are rushing through the level your clear time will be low. This is especially useful when trying to unlock the final figure of Wesker for the "They're ACTION Figures!" achievement or trophy.