How can I survive the flood?

My friend and I had issues as well until I figured out a couple things. I think we barely got bronze, but these tips may help you survive at least.

Use the environment weapons
There's orange circles on the ground with a structure above them. When you stand on the circle you can use the laser weapon, which looks like it one-shots any flood (especially useful for the larger humanoid flood). Standing on the attached circle recharges the laser faster.

Voice communication is key
This was especially helpful for the flood that latch onto your back.

Use your armor powers [LB]
As I recall, one of the classes for the first level comes with the regen power. Pop this down when you're low on health and you should have no trouble surviving, at least while it lasts.

Find weapons you like
I personally prefer the assault rifle/shotgun. The level generally has a couple options, scout around. The first level has assault rifles to the left. Ammo seems to run out pretty fast, so remember these locations for restocking later.

Hopefully these tips can help you survive. We got gold accidentally on one level, but I don't have any strategies for high scores yet.