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.