RAGE

This achievement is obtainable either via Xbox LIVE or by playing splitscreen while the game mode is found on the third disc. There are nine Legends in total and they must be completed one after the other as only one mission is available to start with and only the next mission will unlock after completion of the previous mission. The Legends are basically just remixed versions of missions you played during the course of the campaign with a noticeable increase in the number of enemies and all can be played on Normal difficulty (though you can play the Legends by yourself, the game warns you that no achievements will be awarded by doing so).

The objective of each mission is straightforward so specific strategies for each one aren't completely necessary. Just remember to LOOK EVERYWHERE as valuable ammo types (Fat Boys, Pop Rockets, Feltrite Rounds, etc.) are normally hidden in various places. Grenades and bandages are found laying around as well and seem to be much more necessary for this game mode. Remember to vocalize with your teammate and be sure to share found items with them. There are numerous hidden caches in each mission that you should keep a vigilant watch for, many of which are locked behind doors requiring a lock grinder (these are hidden in the missions as well and cannot be engineered like they could be in the Campaign).

Lastly, simple strategies can go a long way in making a difference for each Legend. Have a teammate lay suppresive fire while you sneak around to the side and flank your opponents. Don't be stingy with grenades when faced with multiple opponents and DON'T try and be a hero and go off by yourself. You only get 2 defibs before your teammate will have to revive you if you fall, and that will take a few seconds. Save valuable ammo types for the bigger enemies (of which there will be several) and don't forget to utilize sentry turrents when an extra bit of fire power can help (such as in the second Mutant Bash TV mission, in particular). Good luck!

In case you haven't been able to find anyone to go through these with, they are all manageable with a second controller in splitscreen (though some are tougher than others).

In general, I kept the second controller a fair ways back, and tried to be relatively cautious. When needed I would run up with it to revive player one and/or clean anyone up that was needed. A few areas won't start unless both people are there, so you will both be under fire. This is where you are most likely to need to use one of your defibs (you have 4 between the two controllers). I revived player two when there was a lull, as he provides a decent distraction. If there is some cover you may want to stick him behind something so he lasts a bit longer.

Life in Prison: Nothing too special here
Season 1: Pilot: There are a lot of arenas where you are forced into close quarters. I mostly used the shotgun and blasted the ones messing with player two until he died, then backed up as I shot the remainder, and revived player two in between waves.
Water Service: This is one of the trickiest, due to the time limit, but definitely doable after a couple of runs for familiarity. You will need to be more aggressive on this one and kill the enemies quickly. I ran up and blasted a few with the shotgun to take them out quickly, then once I went down, followed up with player 2 to mow the remainder down with the assault rifle, revived player 1, and repeated as needed. You will need to use both controllers to disarm the bombs. One holds A to open the access panel, then the other points at the bomb and presses A to disarm.
Trophy Hunting: Lots of mutants, so use your shotgun and refill your ammo frequently. There are two arena fight at the end with some larger mutants and regular mutant minions, I would focus mostly on the small guys and get them out of the way so you can focus on the big guys. A decent place to use turrets or grenades if you have them.
Rusty's Resupply: Another timed level. You don't actually need all the parts so you can grab them if they are handy, but they are just for high scores. Again you will need to be more aggressive to hit the time limit with just one active shooter.
Unwanted Guests: This is the easiest level to do solo in my opinion. I would recommend doing this level fro the nightmare and no incapacitations achievements if you need them. They are all authority enemies, so most are long range. There are a few authority mutants, they mostly come one at a time, but there is a group of three that will come shortly after you cross the open area where a few guys jetpack in. Pop out of doorways, shoot a little, then fall back. There is a heavy enemy near the end, so use a little more care with him, toss a few grenades, etc.
Extermination: This is very similar to Season 1: Pilot. There are more enemies though, so make sure you are keeping your ammo topped up, backtrack and hit up ammo tables before starting each room if possible. You will likely run out of ammo on your favorite weapon and have to use your backup, or switch controllers.
Grab and Go: Nothing too special here, but beware of the glitch if playing online, make sure the non-host goes on the elevators first.
A New Toy: There are a couple of close quarters fights against heavies (sometimes multiples) in this one, so use your grenades freely there.