I did this on the map "Mechanized Invasion" by myself with two controllers on RECRUIT DIFFICULTY. The reasoning behind two controllers is because a dummy player is good to focus some enemy fire on, so you're no overwhelmed by other enemies while trying to take out a more powerful enemy.
What you want to do, is IMMEDIATELY after the game starts, send Controller one (You) out and into the mech under the bridge across from the water creek thing. Get him into the mech and then head over to the mobile home to the northeast from where the mech was.
The dummy controller needs to pick up a weapon crate and get ANY weapon equipped. After re-equipping the dummy, position him in between two walls near the mobile home, it's a tight space near a stairway right next to the mobile home. (The stairway is on one side, the mobile home is on the other, and the space you need to sit in is in between the two.) Go into crouch and face the opening.
Now what I did was just completed every wave in the mech. Things to watch out for:
Wave 4 is your first fight with a mech of your same stature. Really all you need to do with ANY mech, is spam rockets till they overheat, then spam laser till it overheats, then switch back to rockets, (USE THE HOUSES FOR COVER AND RUN AWAY IF YOU TAKE TOO MUCH DAMAGE)
Wave 10 has you fighting a larger mech and a smaller mech, take the bigger mech first, as he is stronger and will destroy you faster. If you can, try to seperate the two mechs from each other by running to a safe area, and waiting for one to follow you.
Waves 11 - 18 are pretty simplistic, just you killing a bunch of weak enemies. However, wave 18 introduces the large teleporter guy with the bolt gun. Take him out first instead of focusing on the little monkey monsters and smaller soldiers. These smaller enemies barely damage you, so it's best to destroy the larger opposition first.
Wave 19 can be difficult. The first time I tried this I died on this wave, because I was in a rush to finish and get the achievement. What makes this wave SO DIFFICULT is that it pits you against two enemies, Rocketeers, and the monkey creatures. FOCUS ALL FIRE POWER ON THE ROCKETEERS, the monkey creatures CANNOT harm you much at all while in the mech.
The Final wave, Wave 20, pits you against several large bolt caster guys (4 or 5), a bunch of the monkey creatures, and one LARGE mech that actually sits on top of one of the bridges. For this wave, focus on killing the bolt casters, and the monkey's first, you can do this UNDER the bridge where the mech spawns, and it cannot even touch you, as it cannot get down from that point. After that, creep out a little till you can see a shoulder or arm of the mech and fire away at it until it's dead, as it won't be able to fire back.
After that, you'll get the achievement.
- At any time your dummy is being attacked, RESCUE HIM. He's vital in case you get downed somehow. (See tip 2)
- AVOID ALMA AT ALL COSTS. Looking at her will either blind you or distort your vision, shooting her will either knock you out of the mech with very low health, or knock you out of the mech and teleport you to another side completely defenseless. During deep fog waves, Alma can be identified as the bright red light seen from a distance.
- If you do get knocked out of the mech, just keep holding X at the front till you get back in. You cannot be damaged as you're climbing into and out of the mech.
- Think before you shoot. The biggest problem with Alma, is that she LOVES to teleport right in front of your fire. ALWAYS make sure she's not in front of you before firing at your enemy.
This video I've made shows you the basic setup to get this by yourself with two controllers: