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Why? Thunderstorm is a fun skull. It replaces low ranks with high ranks and would be a better challenge than most of the skulls they have currently.
░█▀▀ ░█▀█ ░█ ░█▀▀ ░█▀▀ ░█▀▀ ░█ ░█ ░░░▀▀▀ ░▀ ░░░▀ ░▀▀▀____________(˜˜˜||˜˜˜˜||˜˜˜˜˜)_∏______l | --------____.`=====.-.~:________\___|================[oo]|_|||___/___/_/~```|_|_|_|``(o)----------<)★☆★☆★ 5 Star Sniper
I dont really see the need for that..
Also, the waves are balanced so that the enemies gradually increase in rank as the sets go on. I beleive (although I may be wildly wrong) that once you get past enough sets thunderstorm effectively is on.