6, 11 (and 12) is only difficult if you're going in with a halfass team trying to save your good teams for later levels. The jump on 13 is really something else. It's the only one I really planned for, saving an actually good team for it, and still failed to 1 shot it (granted water is one of my weaker elements). The biggest issue, and this applies to all of tower in general, is that you can't really practice the match, so just one mistake and you'll have lost the key and would probably have been better off using an off element to weaken instead. Basically, the best strategy for this tower for someone not op, is to not bother trying for 15 at all, and just use the 2 keys for it to pass 13 and 14 with 2 tries each. That strategy itself suggests that the jump at 13 and 14 is way higher than anything else.