In my opinion the best way to get high slayer is to afk it as much as possible. Therefore I block/skip: nechs, all strykeworms, aquanites, spirit mages, and dust devils. There are a few non-afk tasks, ie mithirl dragons, that I do because they are really good for xp (or money). And I do waterfiends only because I'm still not 99 summoning. I also have a tips and tricks guide about slayer on my youtube channel. I'd suggest you go check it out before you start hard core slaying. The link is in my sig.