pointed out a few posts back, you need a full team of four players (three at a pinch) or things just degenerate fast. I was lucky in that I found a team that was competent and we did well (only failed one out of four missions), but later on I was stuck with a bunch of numpties and it just wasn't happening.
Probably the best thing to do is to split off two-and-two for objectives and all hunker near an ammo cache for the standard waves. It also helps if your chosen buddy is of a complementary class. By which I mean at least one of you needs to be a biotic, because guardians are a major pain unless you can relive them of their riot shields.
It's also vital to long term survival to prioritize your targets. Combat engineers drop turrets so they need to be taken care of asap. Phantoms are also a huge threat if you let them get too close.
Word of warning, twice now I've tried to buy an equipment pack only to have it fail. The first time it was a veterans pack which cost me 20,000 credits and left me with nothing to show for it! Grr.