ok, so, the real question here, is how are you so overlevelled in marleybone? Youre like, 10 levels above anyone else there!
It's very easily done with side quests, side worlds, repeating dungeons to help others or for drops, house gauntlets, one-shots etc etc...even porting ahead for fishing and crafting quests gives additional XP. He could have done Wysteria, Grizzleheim and Wintertusk in their entirety!
The game is carefully balanced to prevent anyone becoming excessively high level for their main quest line. It's possible to be painfully under-levelled and struggling due to only doing mains, but if even if you're the highest end of the scale you can not be actually over-levelled. With levels requiring an increasing amount of XP before leveling up it all evens out pretty fast.