skulls wont give you xp and anything under level 47 for a 60 boosting u will give you significant xp reduction. The only way to boost from a level 60 openworld to a low level like yourself you would need to use lvl 60 pets such as warlock or hunter pets and having someone dead in the graveyard for whatever reason gives you full xp. If you don't want to do this you should boost in dungeons with 1 60 and 4 lower lvls to get the most xp.