Major goal of this level is to locate the sunken ship, and to activate the switches, as indicated.
One part of the level requires you to enter a Devlin breeding pit. Although you can kill off most of the tiny Devlins, be warned that there is an invisible Devlin that will constantly attack you while you remain in that room.
There is a point where you must cross a narrow bridge while being shot at by a cannon at the far side. You can make this jump by first running out to the middle area, and moving to the far left or far right of it. When the cannon fires, it will aim off center, at which point you should be able to squeeze around the shot on the narror walkway, and make it to the door. Don't dawdle at the door once there, though! The cannon can still reach you!
This next room will have a difficult fight. Once you've completed it, the exit will lead you to a ledge near a hallway you've been to before, but instead of jumping down, turn to the left, and go through that door in order to hit a switch that opens the rest of the level to you.
There are three switches at the beginning of the level that control 3 triangle-shaped platforms. These switches additionally control 3 platforms in a room at the top of the level (near the large outdoor area), which, when raises appropriately, will lead you to a secret room with a stash of ammo.
In the room with the switch, located after the 2x recharger and invisible Devlin, there's a ledge that can be grenade-jumped onto, which will get you some additional ammo.
When you are jumping the small ledges in the large outdoor area, there will be one ledge with a hallway that leads to a room with cyborgs. If you move towards this hallway, a rocket launcher and ammo will teleport in.
At the end of these ledges will be an open area where several Pfhor teleport in. On the wall next to this ledge is a secret door, leading to a small room with a model of the spaceship.