You have two options for the tower of fame:
(1) Try to place outside the top 16 every week so that you get max rewards but don't level up to the league where you are underleveled
(2) Drop a rank in the leagues by not fighting any league battles for one week.
Both of these options will give you more exp and other rewards than sucking it out in the overpowered league. Trying to place 15-30 is more risky but does let you compete in activity challenges and win that involve "gain XP"
I haven't gotten to a situation where I need legendary boosters. The other levels of boosters are available as rewards in the arena. Try to time your booster use so that both your leagues and champion fights benefit