Max level in the game is level 40. After beating the game you will be lv 30-34. If you are going for the gold, do those first, otherwise just keep replaying the last level. Last level is just the boss and as you level up he will get easier to battle.
DLC: If you have the DLC "Jungle Hunt", play the last level until you reach lv 40. The DLC Act 5 boss and the pirahna fish provide good XP and you will reach lv 40 very quickly.
Happy Gaming!