By getting this you'll also unlock the other two killstreak achievements:
These can all be done in a local "Bot Match" by setting the AI difficulty to the lowest setting (0/5) and being relatively careful. It can also help to have two extra local controllers so your allies don't steal any kills but the low-level AI is incredibly basic and will often push even when they're low on health. It's probably best to use your most comfortable character but Lonestar is a simple character who can do a lot of damage with upgraded blaster and dynamite.