Do not try to take out any Flood types with a sniper rifle. It will simply go through their bodies without killing them. The weapon of choice is the shotgun. You can take them down with one shot. Remember, the closer they are, the faster they fall. On the small crawling Flood, use your assault rifle in short controlled bursts. Once one pops, it will also pop the ones nearby. Save shotgun and pistol ammunition for the working class Flood to drop them fast. It is also a good idea to hit them with your gun after they fall to make sure they are dead. On the fat Flood, use the shotgun to pop them, then clean up with rifle. When you first meet the Flood, they will outnumber you in groups. For the large Flood, use a shotgun, rocket launcher, or most importantly, a fragmentation grenade. Do not use a plasma grenade because they will run towards you and kill you. When you get attacked by the Flood, after you make them fall down the first time, shoot their head off while they are on the ground. They will not get back up.