Chasm: The Rift

I highly recommend making a save at the reflector powerup before attempting this. You must first reach the boss by going down the right/first entrance at the beginning of the level, and progressing all the way to the teleporter that takes you to the boss. From here, head to the last pillar on the right. On the right side of it, you’ll see a switch to hit. This turns on the pump to rid of the toxic water in the left/second entrance. Make your way all the way to the reflector powerup, MAKE A SAVE, then grab the powerup and make a beeline all the way back to the boss and stand right in front of him. One reflected shot takes him out and completes the game.

Make sure not to skip the credits afterwards for an additional achievement.