This game can be a little overwhelming at first but once you get the hang of it it's cake. First off buy the shotgun: it will be your best friend, it's ammo is low, and it can do brutal damage once upgraded. Next, don't buy the minigun and autocannon; they are basically the same gun. The minigun is cheaper and ammo is less but the autocannon is stronger and slightly more expensive so they kind of cancel out. So take your pick, I use the minigun. Some good ways to survive are to continue moving; don't be still and let the enemy concentrate on you unless you plan on shotgunning them to hell. Pickup anything you see drop and watch out for the corners of the level. There is a glitch I think in the game that will sometimes trap you in the corner of the level and pretty much screw you over. I hope this is helpful, enjoy =)
