This achievement is easier with a buddy, but possible to do solo.
I used the grunt birthday party and extra grenades skulls, because the headshot confetti explosion sets off grenades. This will make packs of grunts much easier to kill.
The Needler explosion is your friend when fighting tough enemies, especially brutes. If you stick 7 needles into an enemy, they will explode. It's an easy way to conserve ammunition. Keep in mind that the elite brutes that use grav hammers are immune to needles, as well as plasma grenades.
Overall, fight defensively. Use cover whenever you can, and don't be afraid to break off and retreat when things get heavy.