The levels dont get any more difficult. I am on Level 125. There are only 2 bosses and they never get stronger, faster, more health, or even mobs added. Always start in the same position and you can take out the bosses in 3 headshots before they even get a chance to fire off. Im rushing to get a Rasengan just to see it but after i get it the gameplay is going to be the same so i am going to stay upset.
My recommendations to keep this game playable for longer
-Each unlocked character has a unique ability (now you have to try them all)
-Each boss level has a different change to it, either multiple of the same bosses or different bosses, adding mobs to deal with too, making the boss faster, etc.
-each weapon has a different stat or trait. (I.e. explosive, goes through objects, fires fast, fires spread shot but you only teleport to middle shot.