Chasm: The Rift

Here’s how to defeat this boss, shoot it until it’s arms fall to the ground and it stops moving, then jump onto the ledges either side of the room then jump inside the mouth of the monster. Once inside shoot it’s heart and you win.

I found it easiest to jump on the ledge and use the BFG type gun to knock it out instantly. Also, you lose all your weapons after this battle.