The flood are real nasty as you know. Plasma weapons and most human weapons are useless. Using a rocket launcher is a waste of ammo and the sniper rifle would be more useful to you for breaking a wall, so how do you kill them?
Well, here are some effective weapons:
*Shotgun (for those of youu who like horror movies)
*Sword (just slices them up and renders their bodies useless to infection forms)
*Brute shot (blowsthem up/slices them)
*Sentinel beam (cuts off appendages/kills infection forms)
*Needler (blows them up really nice)
Use your shotgun only on the combat forms. Use sentinel beams to cut off Combat forms' arms (causing them to self-destruct) or pop infection forms. Also, the sentinel beam is good for annoyingcarrier forms and making them explode. Needler are surprisingly effective and underused. If there is a huge wave of flood coming towards you, frag them, then let rip with dual needlers. since the needles explode, they rip araprt the combat forms, making sure they stay dead. The brute shot is a good last-ditch weapon, but it only has for grenades per clip so use it wisely. Finally, the sword. It is extremely effective (but don't go off trying to kill carrier forms with it) against all types of flood. especially infection forms. If there are a bunch of them and you don't wish to waste grenades, use your sword. Surprisingly, infection forms don't deplete your battery! Also, the sword can destroy dead combat forms.