On any of the Campaign levels, in
single or co-op, this is some good advice:
In a cluster of Covenant, try to take out a grunt or two, or maybe an Elite,
before you throw your first plasma or frag grenade. When you throw the grenade,
try to land it in proximity to the enemy you just eliminated. Your odds are good
that the defeated foe will have dropped a few grenades and they'll chain react
with yours. It helps you get the most bang for your buck out of your 'nades.
Problem is there sometimes isn't much left to pick up afterwards. Works better
indoors rather than out.