Spamm TMQ4 even if it's boring......it goes fast and u get 21k exp....10x tmq4 = 210k+ exp
Or as it was said....spamm with a dw ud3 with exp pots!
54>55 is 1 million exp, 1 TMQ4 is around 4-5 min even with the fastest lv70s, 10x is near 1 hour, u think i can find a strong party to do 50 run in tmq4? i doubt it.
to be honest if you can find someone to help you spam ud3 hard thats going to be the fastest way , each floor [killing the beetles] clear is like 3-4 minutes roughly 20-25 times and you're done