OK, is this maybe one of the Egg Pokemon - Exeggcute or Togepi?

Slow or fast? I can’t decide,

Togepi, like Machoke, starts out slow, but it can get a lot faster when fed Carbos; Exeggcute is 6 eggs, so they may debate with each other how fast they move.

But training will fill me with pride.

Exeggcute is 6 eggs, as I said, so when it's proud of its progress, it's proud x6! Togepi evolves when it gains Happiness, which may be synonymous with pride...?

Physical strength, defensive skill,
Make fighting weaklings easier still.

Togepi has decent stats and HP; Exeggcute is Psychic, and Psychic > Fighting.

I’d be a champ, I’d be a king,
But I’m stuck on a single thing.
I need an aid to see it through,

Togepi and Exeggcute evolve into strong Pokemon, both of which look like they wear crowns (Togetic's spikes and Exeggutor's leaves). But they can't evolve alone - they need a Trainer to either love them or give them a Leaf Stone (respectively).

But with help, I will shatter you!

Togetic, I believe, learns Rock Smash and Brick Break; Exeggutor learns powerhouse moves like Barrage, Solarbeam, and, in some instances, Bomb Voyage.