Deathmatch is the best game mode. You need 10 people to completely fill a lobby. Then select either Asia or Oceania servers. (They barely have any people)
Then scroll all the way down on the server browser until you get to DEATHMATCH. Also from completing this today we think that BUNKER and FACILITY are the best maps, since they are the smallest. Then call out the name of the server so everyone can join.
The rest is pretty self-explanatory: Get 10 kills while the enemy is repairing.
Tips to speed this up: Let one person get all the kills we were averaging 70+ kills for one person in 7 minutes. Also shoot yourself twice with the tow rocket so that the person killing can kill you faster.
The Mech called the Berserker has the least amount of health. Shooting two rockets won't kill you. (You also can buy this Mech to unlock the Reaper and Marauder faster which you need for two separate achievements)