You need to kill any kind of Loaders for this challenge. Loaders are the Hyperion robots that Handsome Jack will sic on players, and they come in plenty of varieties. You'll first encounter them during the A Dam Fine Rescue story mission.
- Rank 1: Kill 20 Loaders
- Rank 2: Kill 100 Loaders
- Rank 3: Kill 500 Loaders
- Rank 4: Kill 1000 Loaders
- Rank 5: Kill 1500 Loaders