The quickest way I found to do this was on the level Mausoleum of Survivor Mode. What you need to do is keep running around the map in circles, grabbing stims, and then grabbing grenade ammo.
Once you have this, just shoot the wall in front of you. Just make sure your not to close or instead of bring you down 2 health bars it will kill you. Then use a stim and wait for the grenade ammo to respawn.
If your playing singleplayer, Ive noticed the grenade ammo takes forever to respawn. So you should probably start letting the aliens take a swing at your health. Then of course use the stims.
Good luck!