An effective tactic when an enemy fighter is attacking you from behind is to do a tight inside vertical loop. You can evade the enemy and put yourself behind the enemy in an attacking position. Timing is important. When the enemy is approaching from behind, wait until he starts shooting then immediately pull up into a tight loop. Pull out when you are at the bottom of the loop flying straight and level. You will be behind the enemy if your loop was vertical.
The most effective way to attack an AA gun is to line up the gun in your sights at a distance where you can see the gun's halo clearly and reasonably close to the ground. The guns will not fire below an elevation of about fifteen degrees. The halo around an AA gun makes it hard to see the AA gun itself but because the halo is centred on the gun aiming at the base of the halo in the middle is generally accurate enough to achieve a kill after a long burst of your machine guns. Fly a straight line directly at the gun until it fires at you. You may want to take evasive action because you are flying on a line between you and the gun, the same line that the AA gun's shell is travelling. Alternatively you may be able to destroy the shell by shooting it in flight.
AA guns automatically fire at you, the player, when you are less than a certain distance from them and at a certain elevation above them. The guns fire at the position they estimate you will be in at the time it takes the shell to travel to your position. Once the shell is fired it continues to travel in the direction set when it was fired. You can evade the shell by changing your direction while the shell is in flight.