Warframe

you need 4 people, who are decent at the game and have good characters,, defense is a fun gametype, you need to defend a set point (or object) from the enemy, every 5 waves you get the option to leave and claim prizes.

for this achievement, you need to get through 20 waves, and then leave after the 20th wave *the 4th time you get the option.

wave 5/10/15/20 are the toughest waves, as they have the most enemies come, i recommend the 1-5 rank level on the planet EARTH, as its the easiest of all level options, and has a good defense point.

its good to have at least 1 rhino, who can curb stomp the map, to delay any issues present.

the 15-20 levels are pretty full of enemies, so save up your energy, so you can use the special things each character can do.

GOOD LUCK!

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.

I have found in the past that Void Tower Defense 1 is the easiest due to the fact that the enemies don't increase in level as the waves go up. This allows you to typically stay in control if you and your group could beat the first 5 waves. Hoping to attempt this soon. In my experience, this makes waves 15-20 much easier than even on Earth (I've only made it to wave 15 then extracted in the past).