DUDE!
take it slow
I had to fight so much to get past that level
your best weapons to fight flood infected forms is two plasma rifles, or a sword. Tank flood, sword or flamethrower. The ranged forms. Are really weak and vaunerable to Needlers. And you have to take it very slow. Creep through that level and when you get the chance drop Deploysheilds in stratigic areas to retreat from ranged attacks. never use the assualt rifle unless cleaning up infection forms. And always keep in mind that sticking the enemy is a bad idea.