18. Duck Behind A Titan Barricade To Auto-Reload Your Weapon
This one is Titan-specific, but chances are that when a number of the opposition were bearing down at once, you're not eyeballing your ammo counter as much as trying to stay alive.
To that end, level up and unlock enough ability points to purchase 'Rally Barricade' (it's the shorter of the two light barriers you can summon by holding circle/B) as when you trigger it, whatever your active weapon is will fully reload no matter how spent the clip is.
Brilliantly, this works with any weapon, from grenade launchers to swords, auto rifles to snipers - just expend your ammo, place down some handy portable cover and hold the line.