So, I've been playing very casually, I have a boosted Troubadour that I haven't played since he was in the teens and I have various characters from 5 to 40... My main is my Necromancer, who I'm finding it very hard to level up, he's currently 96. I'm striking out wiki guides, so any help of dungeons or quest lines I can do with my mercenary would be greatly appreciated!
At level 96, the best way to level up is to run the Altar of Malice signature line. Takes you through all of the overlands and all of the solo dungeons from that expansion, while providing great quest XP. From there, you'd move on to the Planes of Prophecy signature line for levels 100-110, then Blood of Luclin from 110-120, and finally Visions of Vetrovia from 120-125. These should all be soloable, with or without a mercenary. Combat XP basically doesn't exist anymore, so you have to level up through questing. There's a few expansions I didn't name in between those that don't give much quest XP, since the level cap didn't raise during those years.
and maybe you want to find yourself a guild who has the guildbuff, from it you get a bit more experience