This achievement requires you to get to wave 20 and extract in any defence mission.
Now the go to one seems to be LITH - EARTH. I found this to be really annoying however since there are places the enemies can get to and can be missed which at the later waves can prove to be a massive problem.
I found the best mission for this is LARES - MERCURY. It's a level 1-6 bit more importantly the map is completely open and parts aren't difficult to get to like earth.
This can be done solo if your frame is a beast. But I personally did it using 2 level 30 frames. My rhino and partners mag were enough to get to 20 without any problem.
The strategy we used is to each take a side and clear it. For example I took the side next to the stairway and door where they would always seem to come from, partner taking the opposite side. Adapt this strat if you have more frames in the battle.
It goes without saying but make sure your equipment is up to scratch to deal with the later waves! Also keep an eye on your energy levels as things like rhino stomp can really save you if you are getting swamped.
Soon as 20 hits play it cool and watch your surroundings. I found the enemies grew a pair at this point and tried to rush us whilst missile enemies would hold back and fire. But in but in the end we nailed it and got out of there.