Try play the level with another person (Co-Op) which proves to be much easier.
If you insist in playing single, then try follow the tactics below...
1. Try to use more shotguns, swords and plasma rifle. (Plasma rifle is best as it has good damage on flood and also gives good melee)
2. Bombs are also nice, throw when the Flood gathers at one place...
3. Try having one short-range (Sword or Shottie) and one medium range (Rifle or Carbine....If it were me, i would still keep that Plasma Rifle)
4. Flamethrower... Nice and Clean... (If you find those floods hanging on the wall, don't use this weapon, instead use melee or plasma rifle)
5. Bubble shield....Crappy....Use more of Cloaking, Deployable Cover...More helpful....(Flood can still melee you in Buble shield)
6. Rockets....Don't get it...useless...Shotties are better....
7. Melee....Best weapon of all....Dont use it against those big Floods as it doesnt do that much damage
8. Flame Bomb...Best Grenade of all....Limited..so preserve them...
