Both of the higher level achievements in Fallout 76 can be fairly tricky, i.e. hitting both level 50 and level 100. There's a variety of methods you can try for quick XP with some trial and error however, Gameranx and Screenrant have some methods I personally like.
https://gameranx.com/features/id/165685/article/fallout-76-h...https://screenrant.com/fallout-76-guide-farm-bottle-caps-xp/