This is how you beat the level of RE5.
After you trigger the cut scene that seperates Chris and Sheva, you shoot the orange thing in him when he is chasing Chris, then he will grow a second arm and jump over to start chasing Sheva, Chris needs to knock down a boulder so that Sheva can jump to the area that he is at then run up to the central platform to begin the final stage of the fight. Either Sheva or Chris needs to get behind Wesker when he raises his arm up and shoot the orange thing in his back, after a set amount of damage the orange thing will appear in the front of him, do enough damage to said orange thing when it's in front, Wesker will drop to a knee, have Chris run up behind him and restrain him. Keep tapping the co-responding button(s) on screen to finish him off. There will also be an interactive cut scene at the end. But you'll figure it out.
There ya go.