There may only be one Fistule, but most weapons have a heavily camped timed spawn NM to kill. For Chloris you have to try to get claim on Chukwa and kill several other NMs just to pop him. For Glavoid you need to try to get Adze and Minhocao, both timed spawn, as well as killing other NMs for pop items and key items, and then kill Glavoid. There are many more like this.

Personally I hate that any of the weapon stages contain timed spawn NMs, it makes the process much more frustrating when they are heavily camped. But compared to some of the others stages, Fistule is relatively easy.