Find mobs around your level and attack them repeatedly, making sure to use said abilities in the process.
The abilities will level up even if you kill enemies without using them (more specifically, as long as they're slotted), but I'm sure it'll go faster if you do.
Secondary to doing quests, the thing which levels me up the most is exploration. You get a small amount of XP from discovering locations, but if you have completed all of the quests already, then I doubt you will have many more locations to discover to be honest.
But do what emeraldbay said, and find mobs around your level and kill them. If you could get together with some friends and kill slightly higher level creatures, you will level up slightly faster I think, but don't quote me on that.