I would not recommend using effigies on slayer. It may be slower XP than agility, but that doesn't factor in the effigies you gain from slayer, along with the fact you still need to max your other combats. If you check the EffigyScape guide in AOW, it has a list of tasks to block and skip to obtain the best effigies, although you would need to adjust that a bit as you want good effigies as well as good XP.