Here is a new solution, and in my opinion a better one, it is not faster, but it's much easier to handle (not sure if anyone has mentioned this). If you just save this achievement and the Uber Hero achievement for last on your list to collect. You can replay the entire game on 999 mode and it makes it so much easier. It feels like playing the game on the easiest difficulty again. On 999 mode, you start with 999 health, unlimited ammo, on Uber mode, and ALL YOUR PERKS as well as all your weapon upgrades (rockets on assault rifles, super powerful laser, etc) if you completed them BEFORE playing your second run through! I literally just attempted the game last night and finished it moments ago in around 5 to 6 hours total run through on Uber via 999 mode. Again, COMPLETE ALL PERKS BEFORE STARTING YOUR SECOND PLAY THROUGH! Because on 999 mode, Perks do NOT progress. So having those completed first makes the solution below work for you.
So I suggest just play the game on the easiest difficulty (first play through), complete all your assorted perks for each achievement/branch, and then start a New Game (second play through) on the Bonus Mode "999". When you play the first level, the moment you pick up a weapon that is related to the perks, you have the perks, so faster crouching, quicker reloads, decrease enemy explosive damage, etc. But the game gets even easier on Chapter 2 because you get the assault rifle with rocket launchers plus akimbo! I just ran through most levels either throwing unlimited knives (for when commanders are near) or rockets on everything. The big mechs, if you hit them directly with both rockets, go down in 3 to 4 hits, and the heavier guys go down in one or two! Plus checkpoints are very nice, so as your health drops (and it will start decreasing on its own without damage) you can just die and start at the last checkpoint.
Final boss fight (another reason):
To me this is a much better solution because the last level on Uber is just a nightmare (especially the final boss). I beat the final boss while just getting hit constantly (but avoiding the EMP blast) on his first stage (where you shot down the blimps) and then on the second stage (the small room) just die. Started at the check point that is right at the start of stage two boss fight and just don't let go of the trigger for the laser gun. As long as you DONT SCOPE the laser, it will fire non-stop without the need to recharge! If you scope the laser it will deplete in two shots. He went down in less than 30 seconds doing this method.
I hope this helps because I was dreading doing this and Uber Hero achievement.