Do you find yourself constantly
mulling over which 2 weapons you want to carry with you through the various
phases of the battles?
My tip is to practice becoming adept at swapping weapons out for very brief
engagements during the game.
The most common example is like this: Your holding a shotgun and a sniper rifle,
but your staring down a couple of jackals, hiding behind their shields like the
L.A.P.D. I would drop my sniper temporarily in exchange for a plasma pistol and
use it to take down those shields with a few overcharged bursts. When they're
dead, go back and pick up your sniper. You'll have saved the rounds that could
be better used (i.e. up an elite's u.no.whut.) elsewhere.
Simply put, you can include more weapons in your arsenal if you can get faster
at recognizing what weapons you have within reach, and using them creatively to
save your more precious ammo (rockets, snipes, etc).
Plasma weapons work well on covenant against their shields, as well as the
sniper; pistol, shotgun work well against flood and covenant when the shields
are down. I recently have grown fond of the needler against the flood because
they don't dodge the fire. Happy hunting!