Oscar has a critical hitbox, critical hit frames, and takes damage at different ranges with a potential to die in 2 Rifle Shells.

You have to shoot him in the mouth, when he raises his legs, and at a specific frame during the animation.

It's easy to manipulate these conditions by getting Oscar to ram into the support beam, back off, and raise his leg. Note that you have to strafe out from behind the column with L1 + R1 + analog stick to hit him; you can't stay behind the support beam, otherwise it'll eat your bullet.

Kevin should have an extra 2 .45 rounds for this setup in case the 2 rifle shells don't kill Oscar (99% probability)