If I Had a Rocket Launcher, I'd Make Somebody Pay is the fourteenth level of Marathon 2: Durandal and the first level of the chapter Durandal. It is the first level in the game that doesn't take place on the planet Lh'owon; you're on Boomer, the Pfhor scout ship Durandal stole at the end of the previous game. After Durandal wiped out half of Pfhor Battle Group Seven, Boomer was critically damaged and it landed on Lh'owon's second moon. The Pfhor have begun boarding Durandal's ship, and you are required to repel them.
The objective is to reach the end of the level alive. This won't be easy, as the level is filled with tons of Pfhor Troopers, Hunters, and exploding BOBs, making it one of the hardest levels in the game. While most BOBs are enemies in disguise, there are a few friendly ones as well.
There is a pattern buffer in the starting room, along with two 3x shield rechargers. Just southeast of the first blast door is another pattern buffer, next to a 1x shield recharger. There is an oxygen replenisher in front of the second blast door; the third pattern buffer is found beyond that door to the southeast. A 2x shield recharger is located in front of the third and final blast door in the southwest corner of the level, and the final pattern buffer is found just behind the door.
While several hidden WSTE-M5 Combat Shotguns could be found in earlier levels, this is the first level in which they can be found out in the open. As the level's name might suggest, this level is also where the player is introduced to the Rocket Launcher.
Completing this level is relatively straightforward, as it is more of a combat level rather than a puzzling one. At the start, be sure to check around the corner to find a couple of 3x shield rechargers. If you're not yet dual-wielding shotguns, pick up the shotguns at the north end of the room.
Use the pattern buffer, then access the terminal. You will be teleported to a room with infinite numbers of Troopers pouring in from several elevated ledges. You'll also see some BOBs, but be careful—these BOBs, and many others on this level, are actually Simulacrums who will run up to you and explode, so be sure to shoot any BOBs that run toward you.
Hit the switch; the blast door to the south will slowly start to open after a long delay. You'll have to survive a while here; use the shield canisters lying on the ground when you're low on health. When the door is open, quickly grab a shield canister and go through before it closes again. Fight off the exploding BOBs here; just to the southeast of the blast door you'll find a 1x shield recharger and a pattern buffer.
Go south to a Hunter-filled area. Turn east to find a switch next to an oxygen replenisher; this switch will (slowly, and after a long delay) open the east door. There are also some shield canisters in this area to help you survive the onslaught of Hunters.
When the door is open, run through. Some BOBs should teleport in; these ones are friendly, non-explosive humans. Go east, then south to find a pattern buffer. Follow the path south, then west until the path splits; go north and run along a large lake of plasma. Go up the stairs and down the hallway. On your left hand side there's a platform jutting out onto the plasma lake; you can pick up a Rocket Launcher at the end of it (map location A), along with a bunch of ammo for it and a 3x shield canister.
Turn around and proceed down the hallway until you come to a third blast door. Push the button and wait; you can make use of a 2x shield recharger here. Once the blast door is open, go past it to find the final pattern buffer and the exit terminal that sends you to Sorry Don't Make It So.
At the east end of the plasma lake with the rocket launcher platform in it, you'll find a submerged tunnel leading to some stairs that take you to a ledge overlooking the lake.
There are a few corridors running through the level that connect the various enemy-filled upper ledges you come across. You can grenade jump onto a ledge to get into these corridors, allowing you to skip a large part of the level.
The first terminal is a message from Durandal, who explains that the Pfhor have penetrated the hull. He explains that the ship is helpless on Lh'owon's second moon; engines, weapons and life support are offline.
This terminal contains the first message from Tycho seen in the game. He says that his attack has just begun and that your situation is hopeless. He says you may still "see the truth", but that Durandal is too stubborn to do so.
This message from Durandal is partially corrupted, illustrating the ship's computer systems are being infiltrated. Durandal says all human defenders near the hull have been routed, and he urges you to fall back and hold off the Pfhor while he tries to evacuate as many S'pht and humans down to the planet as possible.