Updated: Dec 21, 2022
At level 87 magic, you can start enchanting onyx bolts and if you 1 tick these, you get better xp rates than bursting and barraging. At maximum, so if you don't miss a single gametick, youll get 580k xp per hour.
Its very hard to do this consistently though, so youll get around 500k per hour. This is a method that you can do while you train other skills like agility since the bolts and runes are stackable.
But since 1ticking is a lot of work, the best other method is barraging or bursting in the monkey madness 2 caves.
Doing this can get you 400k magic Xp per hour, which isnt too much less than enchanting bolts, and youll also get hitpoints experience. Both barraging and enchanting onyx bolts cost around the same.