So just checked Mawile's stats, and Mega Mawile (Assuming it only raises Sp. Def and Def like the site says and not all stats) will basically be tied with black Kyurem for second highest base attack once we factor in Huge Power which doubles it's attack stat from 85 to 170. If only it's base speed wasn't so abysmal, it would probably be able to sweep even uber tier threats, especially since fairy is good against Dragon
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Thought about it more and realized that since it's EV and IV boosts would ALSO be doubled by huge power, Mega Mawile essentially has the potential to have the highest attack stat in the game by a LOT (The highest right now is Deoxys attack form with 180 base attack and a potential of 504 max attack. Mawile's current Max attack is 295, so doubled by huge power, it's max attack would be 590)
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And it learns Sword Dance, so if given a turn, it can raise to 1180 attack. Lovely. It also learns sucker punch as a priority move so it's speed won't always be a problem. Now imagine if it was able to learn some new fairy type priority move.