Enter The Gungeon

In the Marine's past, you have his sidearm and a Hegemony Carbine. You'll be fighting the Interdimensional Horror, who is quite the bullet sponge. You may run out of ammo for the Carbine near the end of the fight. You also have a group of marines at the bottom of the room who will fire on any targets. The ID won't move normally, but can teleport after attacks.

The ID has two main attacks. The first fires all around him with ample room in between. At the end, a ring of tight bullets is sent around that must be dodge rolled through unless you're on the other side of the room. The second fires squiggly bullet lines reminiscent of Ammoconda. You'll be able to juke most of these, but you'll likely need to dodge roll past the last one or two.

The ID is also capable of summoning mobs. They move and fire sporadically, but will try to predict your movement to box you in. Additionally, the room may change to a more alien, Doom-esque appearance. Avoid the orange floor, as it is poisonous.