I hate to waste prayer potions on slayer quests. I found a nice way to avoid it while killing Terrorbirds (lvl 81). In the second lower level passage (or whatever you call those things), there are two terrorbirds and a tortoise. When you are attacking one, you only need to have protect against ranged on until the other terrorbird advances up to the tortoise. Then he will just sit there and stare at you. If you want, you can just wait for your latest victim to reappear, kill him again, let him reappear, kill him again, etc. The whole time the other crazy little bird will just sit and stare. Eventually, the second terrorbird will disappear and respawn. When he does, it's only a matter of time before he starts attacking you again. However, it only takes a few more seconds of prayer and he will be up against the tortoise staring some more. As far as I can tell, the works on both of them, just pick the one you want to kill. I haven't tried it anywhere else in the sewer, but it might be harder to do with more than two birds. I does save a lot of prayer potions though.