Fracture

The Rhino is first given to you after the first fight with the big genetic reject (although I don't remember his name). It is a giant rock attack that will bowl over enemies and typically kill them in one hit, but they will rebound and hit you too, so watch out. You need to accumulate 25 kills with this weapon.

Note: Gun kills apparently stack even after death or loading from a checkpoint. If you find a good point in which you are killing many enemies with this gun and you've just saved, press start and select "Load Last Checkpoint" (or whatever it is). You'll reload from the last checkpoint but the game's internal counter doesn't. Thus, the amount of kills you had with this gun prior to your reload will not return to the point of that save.

Can be done on multiplayer

Game type - Break in
Map - Ft. Point

In the host menu press x for additional options and scroll down to weapons press a scroll to select weird stuff press a again and start the match. You will have to look for it but it's now available. Get your kills and it pops.

Load the level "Recon" on casual and proceed through the level until you fight a giant mutant, it is about 2 minutes into the level. Shoot him in the face, chest, and back until all patches are red. When he dies he drops the Rhino. You can kill the two enemies that spawn, but they are kind of a pain because they burrow. I recommend saving your ammo and getting in the vehicle and advancing through the level until there is a checkpoint and a force field blocking your way. there are 2 enemies behind a blockade that you can kill very quickly and reload the checkpoint to kill again, ect.

Alternatively, if you have 80 data cells you can just load up the weapons test area and gain kills that way.