This achievement can be had utilizing the same method NE0182 mentioned in the "Scrapped" achievement.
- Load up a Private Escalation game.
- Select "Obsidian" for a map
- Make sure your character has the "Thunder Blast" ability assigned
There is a small section where Melee enemies are unable to reach you. You can still be hit by gunfire, and projectiles; however you should be able to die, and rinse and repeat to build kills toward this achievement. You do not have to complete the wave in order for kills to register. It should be noted that this achievement is for number of multi kills - each individual enemy killed does not count towards this number. So if you kill 5 enemies at once, that is 1 multi kill, which is the same as if you were to kill 2 enemies at once, registering for 1 multi kill. So you need to kill 35 "groups" of enemies, not 35 enemies total.
Watch the video below to see the correct spot: