Act 5: C2 - Desperate Stand
Kill the reaver in the front, which shouldn't be hard at all, then follow your team to the platform. Just use the lancer from a position and after a reaver lands, a chopper will clear it out.
After that, go right and when you see some steps on the left, follow them to some ammo and a report. And from here you can attack the reaver and drones on the second platform. Follow your team to a part where you mow down enemies below. There are frags and mulchers behind you at the wall, if your position allows you to aim low with them. Otherwise just use the lancer, hot bullets, and the ammo to clear it out. When reavers land to the left, just hide as a raven takes them out.
Then go across the bridge and up the stairs, no need for a mulcher, and take point on the second turret; I think the first turret causes a glitch, not sure. Anyway, on here just kill the reavers that land, and you can try hitting the ones that don't either, they count too. Kill four and then head to the next area.
There is a mulcher, but first go all the way right, down the steps, and pick up the COG tag. Save that mulcher for later, or save a lot of its ammo if you want to use it, and just use the lancer on the drones below. Perhaps use some of the mulcher ammo on the grinders, but you really could let your team take them out. Now use the mulcher when the two brumaks appear, and remember to aim high at their guns.