If you are trying to destroy aircrafts that don't stop moving and turning in all directions, and really want them down, here's what I do;
1- stabilise/put the plane back to horizontal
2- increase thrust
3- when you think you are far enough, pull the analogue stick so that your plane makes half a looping. Not a complete one, remember
4- your enemy should have followed you, so he will be heading in your direction. If he is far enough, stabilize your plane
5- increase or decrease altitude to be more or less at the same height as him, but do not incline the plane too much. One thing I do is turn it upside-down, so it falls automatically. correct also your your heading, in case it isn't right
6- arm suitable weapon, and wait for distance between you and the enemy to be quite short. If too close, he the missile wont hit him, if too far, he will have time to dodge
7- fire
Remember, re-increase thrust after the looping!
Usually, several planes follow you, so you can shoot them all down, and if you see that when you are aiming the enemy, and they are aiming at you too, don't wait for distance to shorten, fire when locked, and steer out of danger!
It always work for me, on any difficulty, if you don't manage to do it, there might be an error while you tried to do it, or maybe there's a different AI in the game, or simply, maybe I missed a step...