Hmmmmmmmmmmm...

I think I've got it.

It's Wobbuffet.

The victor thrice, approacheth hence,

In Pokemon Fire Red/Leaf Green, you have to win against all 3 of the games major quests before you can access Wobbuffet in the Cerulean Cave: Defeat Team Rocket, defeat the Elite Four, and conquer the Sevii Islands.

The door hath been blocked without sense.

Wobbuffet is found in Cerulean Cave, which is blocked by some jerk until you finish everything lese in the game. Jerk...

Façade awaits you, keyholder,

In Ruby/Sapphire/Emerald, Wobbuffet's baby form - Wynaut - can be found in abundance on Mirage Island; the key to accessing said island is a certain Pokemon (which I sadly don't know :p).

Beware thy future batt’ling err;
Seek wisdom now, or fate is doom.

Picking a fight with a Wobbuffet for no reason is a bad idea. It can force you to battle it thanks to its Shadow Tag ability, it'll blast your own attacks back at you with Mirror Coat and Counter, it can shake off Burns, Poison, etc. with Safeguard, and if you do manage to weaken it enough, it'll use Destiny Bond on you. The only safe way to beat it is to use a OHKO move on it...

Light sheds as growth avoids its bloom.

Wobbuffet lives in dark caves so that its shadowy tail grows. A Wobbuffet's tail is as important to it as whiskey is to Ted Kennedy.