Before you begin attacking the Nemesis you’ll want to do a loop around the roof to find some Explosive Rounds, Green/Red Herbs, and finally some Shotgun Shells. In order to take him out you’ll need to make use of your most powerful weapons, which are the Shotgun and Grenade Launcher, use these to focus fire onto the canister on his back until it breaks and then continue firing at his Chest until he finally goes down. If you have any Grenades at your disposal, now is the time to throw them, as one will bring the Nemesis to the floor.
Whilst circling the arena and slowly dealing damge to him you’ll want to make use of the generator on the ground. Wait for the Nemesis to approach it, then fire one bullet at it to stun him momentarily, this will recharge over time so you can use it again.