Act 1 of the game is called the "Hall of Heroes" and is made up of 3 sections: 1-1, 1-2, and 1-3. This has to be completed in single player or Co-op to be unlocked.
The boss at the end of 1-3 is called GRNDL-1. You can take the slow approach and shoot at him while moving backwards and then rolling out of the way when he swings at you, or you can just dash into him and attack with everything you got.
Every now and then he will run off to around the outside of the room and shoot some flying robots at you. If they stick to you, they will slowly drain your health, so roll to get them off.
After he has taken so much damage, he will walk on his rear legs after you. Either aim at him and shoot, or jump and attack his head/chest. Be careful, though, as if you get too close there is a very good chance he will grab you and press you against the rollers on his chest, doing you damage.