The Marathon Spoiler Guide - Eat It, Vid Boi

Again, in the first room of the level, let the tanks guys go fight each other with fancy maneuvering. Also, if you stand outside the room, you can fight the one or two aliens that exit it easily.

To continue on from this room, there is a darkened section of the wall that is a door. In this room is a switch that will activate one of the elevators in the other room, allowing you to continue on.

Alternatively, instead of first letting the tanks and Pfhor kill each other, you can make a run for the door, charge up, collect ammo, and save, then you can hit the switch, leave the small room and ride the elevator (still avoiding the tanks), enter the cross room to activate the S'pht to follow you back into the tank guy room. You should wait in the side room, poking your head out only once in a while once a battle ensues. Done properly, the tanks and the S'pht will fight each other, leaving you to clean up the weaken remaining enemies.

Most of this level is exploration, and hitting any switch that you come across. The goal is to raise the path across the lava in the first room, and to open the doors on the other side of the path.

The fusion pistol can be obtained in the lower right hand side of the map. You'll need to raise a platform out of the lava with the switch in the same room, and then jump across to the new platform to grab the ammo and to recharge to 2x shields.

The shotgun and ammo is located in the center of the 'cross of lava' room, although it will be under the lava at the start of the level. If you really need it, you can make it with 2x shields by dropping drop, grabbing the stuff, then running towards the opening that is down there, making a quick turn to the left, and jumping on the elevator that is located there. However, it is a bit more accessible when the lava level is lowered.

Secrets

In the room with the save terminal, shield regenerator, and terminal, there are three small elevators. Two will take you to a 2x shield regenerator, and a third will take you to a good ammo stash, including shotgun ammo.