Legends vs Strategy

A very common notion in the competitive scene (the 'big shots' in online battles) is that legendary Pokémon destroy everything there is.

What if I told you... This is both true and false.

First off, why it's false. Even though legendaries have insanely high stats, most users of non-legendaries don't quite seem to understand how to pull them off properly. I've had many a battle where my opponent sends out two intimidating beasts that, due to being legendary, should surely secure victory for them... And then I score a super-effective hit on both of them, and now we're 2-0. I've already won either a third or a half of the battle. Oh, and two more legendaries replace them and I get a 4-0 or 6-0 sweep.

What gives? Aren't legendaries giant beasts that are made of pure awesomeness?

Well, it's just that so many common Pokémon counter them. While Arceus has a base stat total (an approximate measure of power spread over all of a Pokémon's 6 stats) of 720, and Mega Mewtwo has 780, non-legendaries aren't far behind at all. Slaking has 670 stat total, making it ridiculously powerful if played correctly, and 'Pseudo-Legendaries' like Dragonite and Garchomp have 600. Also, the exact distribution of the stats comes into play, making certain Pokémon far greater sweepers than legendaries.

On to the next thing: The idea that a legendary secures a win is entirely false. Someone who believes that legendaries automatically win is less likely to have a good strategy, and probably even less likely still to have a good counter-strategy. Mewtwo is easily knocked out in one turn by a Greninja using Night Slash. Yveltal and Xerneas are generally not even that threatening when used by an inexperienced player, and Smack Down or Poison Jab, respectively, will make short work of them. Arceus is quite a special case, as it can be any type. However, the rarity of Arceus, and how it's rarer still to see Arceus played well, make this less relevant. As a general rule, any super-effective hit with a move of high enough power should either faint or greatly threaten Arceus, and it probably can't fight back effectively unless its four moves are diverse enough (of course, diversity is rare on a legendary that isn't played to its fullest potential).

And of course, type matchups. Mega Mewtwo could tower over your entire team, most likely being able to knock them all out in two hits. And then a fast sweeper Greninja comes in and uses Night Slash (Y) or Acrobatics (X). The same goes for all legendaries, and indeed everything. Everything has a type weakness, and everything has a counter.

So legendaries are useless, right?  RIGHT?!

No. If you plan ahead, think of your strengths and weaknesses and teach moves and distribute EVs and develop a strategy that takes advantage of your strengths and nullifies your weaknesses, and lets you know when it's a good time to switch to something that will actually survive this next hit, you can make something rare and valuable - a legendary that actually wins battles.

And yes, these rules do go for everything. an insanely overpowered beastly destroyer of worlds can easily be beaten by Slurpuff.  And then the Poison-types come along. And then Claydol says hi.
Also, this is how the metagame evolves. Counters are developed and the game as a whole becomes more diverse. If you're a competitive battler, you should keep this in mind, and mix your team up a bit! Who knows? Maybe Mega-Mewtwo will become the anti-metagame in the future. Your team might become the next anti-metagame! :D

-Pokéxplain