Currently the most challenging levels are ones with multiple paths to the goal, since there seems to be some expectation you'll have heroes that can attack universal targets. I have plenty of allies that can attack anywhere on the map but are ineffective vs. MOABs or lead Bloons. Of course the game recommend buying a DLC hero for this but I welcome the challenge of working around it. Just got a legendary weapon for Pirate Cassie that just deletes everything on the stage once every three rounds. Feels a little OP but made extreme mode a breeze on any maps that allows water heroes.