(sighs)

I should've guessed this from the start...

Is it Articuno?

From ages past, I’m the defender.
Articuno has existed since the RBY Generation, and probably much longer, since it is a Legendary. As for the defender part, it protects and guides people lost in icy mountains.

My strong attacks will make you shiver.
Articuno is an Ice type.

I own the greatest move of all,
Articuno learns Sheer Cold, an instant-knockout attack that no Pokemon can resist...

Though to get it I have to stall.
...But not until LV. 85; that's a long time even for a Legendary Pokemon.

Don’t crack under the tough stress
Articuno has the Pressure Ability.

Or you’ll fall to the battling best!
Like I said, Sheer Cold instantly knocks out the opponent, it can't be resisted, and it only has two drawbacks: it's poor aim (which Articuno counters with its Mind reader attack), and its inability to beat higher-lever Pokemon (Like there are that many LV. 85+ Pokemon in the games, anyway.).