To obtain this achievement you must kill 1000 grunts! Good news is there is easily over 1000 grunts across all 4 halos. It is worth noting that if you die and kill the exact same grunt every kill counts as 1, meaning that you can kill 1 grunt 1000 times and obtain this achievement! (Which won't be necessary).
If you want to make your play experience more fun fighting grunts you can put on the GRUNT BIRTHDAY PARTY skull on! this will make it party when you headshot the grunts. If you plan on playing this game a lot do not even worry about this achievement.
If you plan on maybe just playing through all campaigns and stopping you might wanna make sure to try to kill some grunts because skipping them all won't get you any progress towards this achievement.
Note: If you kill the elite usually with them then they will freak out and start running away from you or around in circles. Can make them easy targets or annoying when trying to head shot them.
Good luck and have fun! Happy hunting!
Video below is a (somewhat) educational and hilarious video done by Machinima.