If people have trouble fighting against the AI P51P try this:
When doing the dogfight mission you always start with an energy advantage, but the AI never spins and retains energy in zoom climbs. Try to stay above him and try to move behind him not towards him, and when fighting in the vertical as soon as you dive back to earth and got some nice speed(above 600kp/h) you can really turn nicely back to a climb.
Don't use curves on the pitch axis, keep the input as linear as possible. You don't want any sudden acceleration in pitch if dogfighting becomes frantic. And as soon as your plane starts shaking, just ease on pitch input.