Before anyone asks, I actually love Aegislash's design. Sure, it might yet be another sapient piece of equipment but a haunted sword? Who wouldn't want that accompanying them on their Pokemon journey? The Steel/Ghost type is immune to Normal, Poison and Fighting. Additionally, it takes a quarter damage from Bug types and is resistant to Flying, Rock, Steel, Grass, Psychic, Ice, Dragon AND Fairy. Which, all things taken into consideration, sort of makes up for the fact it's susceptible to attacks from Ground, Ghost, Fire and Dark.
But while it has laudable typing, Aegislash's popularity primarily hinges on its stances and King's Shield. In a nutshell, Aegislash begins every match in Shield Form, which provides an impressive 150 base defense and 150 special defense. It is only when you choose to have Aegislash go on the offensive does it switch to Sword Form, swapping its high defense for high attacks. King's Shield makes things even more interesting for Aegislash. A Steel-type move, King's Shield protects from all physical and special moves, drops the opposition's Attack should they come within contact with the Aegislash, and reverts Aegislash back to Shield Form. Best of all? King's Shield always goes first in every round.
There's more to Aegislash, of course, but everything else is pretty much window dressing at this point.
There is unpleasant and there is so absolutely offensive you need to kill it on sight. Greninja is a crowd favorite not because he's the frog ninja with a tongue scarf but because of one crucial ability: Protean. Protean is, to put it mildly, totally bonkers. It's an Ability which changes Greninja's typing to fit any move it currently is using. In short, STAB attacks everywhere. Everywhere. The fast-moving amphibian, who rocks one of if not the highest base Speed in the game, basically deals 1.5x damage with, well, everything its arsenal. To make matters worse, you can also utilize Protean defensively. Raichu looking to cook itself some frog legs? No problem! Go ahead and Grass Knot it up. Laugh at the Thunder Bolt Greninja is a nightmare waiting to happen, a flexible frog who can be easily used to lead a furious charge against the opposition as either an offensive Sweeper or a damage-over-movement-dealing Spiker. But mostly as a sweeper. Good god, he's a nasty sweeper.
(Note: The Pokemon in question does have appalling defense and negligible typing. Don't expect it to tank, well, anything.)
Okay, okay. There's no delicate way of putting it. Mega Mawile's a bit of a cheap bastard. The Steel/Fairy, which naturally enjoys some killer resistances, is a brick wall and Chuck Norris rolled into one. With Swords Dance in its arsenal, Mega Mawile can potentially dole out some catastrophic amounts of damage. How? A high base attack combined with the attack-doubling Huge Power and shameless abuse of Sucker Punch, that's how. To make things even worse for the opposing Pokemon, Mega Mawile is armed with pretty solid defense.
Grousing aside, what's really great about Mega Mawile is the simple fact that it isn't necessarily broken. Against a newbie, chances are the innocuous-seeming Pokemon will probably break faces -- especially if it's allowed to run amok and Swords Dance to its heart's content. But Mega Mawile's weaknesses are easy enough to exploit. Fire and Ground aren't uncommon to find on the battlefield. Combined with Mega Mawile's shrug-inducing Special Defense, laughable Speed and a handfu of other variables, it makes it far from impossible to...
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