|Spider Snipers and Regenerating Flood: Spider Snipers are a real hazard throughout this level. They crawl on ceilings and walls, deploy, and then fire at you a green/yellow beam (actually kind of a Needler crystal from their tail). If a few of them are firing on you at once there is little chance of survival.
The only way you can kill them is to use Flames, Grenades, the Needler, or Melee attacks. Anything else will make them go into their shell which does have the effect of them not firing on you for a few seconds.
In general, as you make your way through this mission, you'll want to take out any immediate threats, like charging Flood, and then carefully and strategically take out the Spider Snipers.
Speaking of Flood, pay attention to Flood that you shoot. Even though they may be on the ground it doesn't mean that they're dead. If you see their body blow to bits, say, with a Brute Shot hit, they're dead. However, if you don't see this it's a good idea to Melee attack them while they're on the ground to finish them off.
Again, Flames, Grenades, the Needler, and Melee attacks are the most effective attacks against them.