At the end of Chapter 5-3 you will engage in a boss fight with Wesker And Jill. You have 7 minutes here to inflict damage on Wesker to make him retreat, easier said than done. Best head into this one with a Magnum (even better if it's upgraded) to do the most damage.
At the start of the fight head towards the blue door on the right and Wesker should give you a heafty kick through it. Make sure your map is enabled, run away up the stairs and Wesker should follow looking for you. You should see his triangle looking both ways when you've lost him, at this point pop out and try to put some lead into him. Might take a few attempts to get this hang of this but it's doable.
Can be obtained in co-op even if you're playing as Sheva as long as the other person gives Wesker a kicking.