From the main menu go to Academy>Training Mode>Play. Once the match has started, press
to adjust options. Pick any weapon you like, turn on infinite ammo, turn on deathless, set bots to recruit, anything to make it easier, and you will get this achievement in no time (I think it took me 10/15 minutes, I'm not sure).
There's no need to play online or even be good at Halo!