I know it's a little late, but there is no mention of the fact that if you get 3 stars on a level with Monster Power set to 7 or higher, the game will give you a level up. This can only be done once per level. Since it keeps your XP the same, but just increased your level, this is best done late in the game, as skipping a level with 2000 XP is not that hard to earn, compared to a level with 90,000 XP.
And I was wondering, is there a max level? I haven't hit it yet, but I also don't have all the skills maxed yet. I didn't know if the max level was when every skill is maxed, or if the max level is lower, forcing you to decide between certain skills.
Sorry to answer my own question but I finally grinded my way there.
Max level is 81, which is when all skills can be maxed out, so you don't need to pick a certain skill set, you can actually get all of them. This also means that if you are waiting to complete the "Difficulty above 7" levels to gain a free level, they should be done no later than level 67, or else you will hit max level before getting all of them. Ideally you should do it at level 66, since by the time you complete all the levels again on a difficulty of 7, you will be very close to leveling up again.
Hope this helps anyone who hasn't played/beaten it yet. I know you can definitely beat everything and get all the quests well before this, but I know some of you are also Completionists like me and want everything maxed
