If you have some really good friends they could get things down to low hp, then if you get the last hit on it, you get the slayer xp, without all the combat xp. Definitely worth doing Back to My Roots, a few k slayer xp every couple of days. Other than that, just use every genie lamp/book of knowledge on slayer. EDIT: I'm pretty sure the last hit gets the xp, it definitely gets the killcount.