OK, then... Is it Machoke?

Slow or fast? I can’t decide,

Machoke starts off with a pitiful 40 Speed, but it can grow to over 200 points when fed Carbos (or whatever).

But training will fill me with pride.

Machoke likes admiring itself in the mirror - that's pride, if I know anything about pride.

Physical strength, defensive skill,

Fighting Pokemon are known for heir superior Attack and Defense.

Make fighting weaklings easier still.

This is what confused me - I thought you meant fighting weaklings, as in 'weak, Fighting Pokemon,' so I leaned towards a Psychic or Flying type. But now I see you meant fighting (the verb, not the adjective) weaklings, which is a pretty large group compared to Machoke's family.

I’d be a champ, I’d be a king,

Machoke evolves into MaCHAMP, the KING of Fighting Pokemon according to most.

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

Machoke has to be traded before it can evolve, so it does indeed need an aid.

But with help, I will shatter you!

Machoke learns Brick Break and Rock Smash via TM/HM ('with help'), two incredibly powerful, groundbreaking (literally) moves.