How to win with a “weak” Knight?

I would suggest 19+1+1 halflings, 1 wild boar, and a stack of skeletons (if there are more than 20 skeletons, you can use two equal stacks instead of one single halfling).

In the first round, use speed on the 19 halflings and move the wild boar forward.
The halfling eliminates about 8 centaurs, meaning one full stack of skeletons moves forward, and the 19 halflings die, let's say, from 4 shots – although this is not certain.
In the second round, use speed on the skeletons if they weren't positioned opposite the enemy – although you can also position them opposite.
The wild boar covers 2 stacks of centaurs, and the skeletons eliminate 1 stack. One centaur remains free and shoots at the single halfling... after that, you can either save the halfling or the wild boar.
If there are about 50 centaurs, only the skeletons survive, plus maybe a single unit. If there are 25-35 centaurs, then several halflings will also survive, along with everything else.
P.S. The same applies to a Knight with his native troops when breaking through archers with greater speed than his own archers – in the first round, it's better to use speed on the archers, and in the second, on pikemen or other units with medium speed