Roll a tank (sorry, I had to). Otherwise roulettes. Also, protip: remember your exp boosts--rested exp buff, food +3%, and the scrolls that increase exp gained from killing monsters by 15% that you get from your Grand Co. flunkies as a reward for some lv 45-50 missions all help enormously. Lastly, your challenge log is a fantastic way to tie in your roulette exp with the weekly challenges such as giving out comms and doing random dungeons. Fate challenge log completion is good ,too.