The little trick in my video should help you get 1000 explosive kills in no time at all. You should have a second controller for this. It isn't required, but is highly advised.
Match setup:
Team Killcount (Deathmatch)
Map: Desert, small variant
Custom weapon setup:
In slot 1 - 4 add shield
In slot 5 add the Plasma Rifle
Bot setup:
Bot count: 14 (to allow your other player to join)
Difficulty: Agent
Bot teams: Humans vs Bots
Personality: Bully
Set the time / score limit to whatever suits you, just make sure you don't set the time limit to infinite (the game won't be able to end properly and the kills won't count toward the achievement.) 20 minutes with no score limit will get you over 500 kills.