In general: simply practice. Try different things, see what works. Different ways of managing altitude, different ways of maneuvering when the AI is on you, different ways of trying to get a good angle on the AI. Eventually, you will gain a "sense" for what is the right thing to do, and what is the wrong thing to do.
But, there are a few things in specific that I've found to work well (in UP3.4, same AI as 4.13.X, may be transferrable to BAT):
When the AI is Behind you, strategy 1
The trick here is to know what kind of turn the AI is programmed to calculate the lead of.
Before update 4.11, the AI generally shot in a way that compensated for your direction of flight at the moment of the shot being released. This meant that when you turned, the AI wasn't able to calculate lead properly, because by the time the AI's bullets had gotten to you, your direction of flight had changed.
With 4.11 or later, the AI now compensates for changes in your direction of flight (turns), but, it's not quite smart enough to compensate for changes in the rate of turn itself. So, to avoid AI shooting at you, you not only turn, but change the way that you're turning constantly. A spiral roll makes it extremely difficult for the AI to hit you.
When the AI is Behind You, strategy 2
Because the AI is good at calculating lead for turns, but not good at calculating lead for turns that are constantly changing, the AI can be very easily fooled simply by you turning left, then right, then left, then right, etc...
If you manage to pull it off correctly, you can make the window of the AI calculating that it's the right time to shoot so small, that the AI lets one or two shots at you, or even doesn't shoot at all, because by the time it's set up a proper angle to shoot at you while turning right, you have already started turning left, and likewise.
Generally, whenever you have an AI behind you, you want to maneuver such that by the time the AI has set up an angle to shoot at you in one direction of turn, your turn has already changed to a different direction.
Head-OnsThe AI can be a very dangerous opponent in a head on, because it has lazer superman aim in that situation. The solution is to roll upside-down relative to the AI, and pitch up. For instance: if the AI is rolled 45deg to the right (which would be your left), you are rolled 135deg to your right. This works because during a head-on, the AI will usually try to roll so that you are above its line of flight, and then pitch up to point at you. Forcing the AI to always be in such a position that it will have to pitch directly down to shoot you makes it almost impossible for the AI to hit you in a head on. Remember: in a head on, it is better that neither of you take damage than that both of you do.
This strategy can also work against AIs that are behind you, but watch your altitude very carefully.
Managing 1v2s and 1v3sYour main priority in fighting against multiple AI at once is to always present a threat to the AI that is closest (time-wise, not distance-wise) to being able to get a shot off at you. Whichever AI is in the best position to get a shooting angle at you, you want to be pointed towards that AI so that you can force them to pass by you in a head on. Once the threat is gone, look for the next one. If you are lucky (you will get lucky with enough time), you will be able to get a clean shot on one of them, and if you're patient enough, you can whittle them down from 3 to 2, 2 to 1, and finish off the last one.
Tight TurnsSometimes it is the case that the AI is making a very tight turn in your direction. In this case, the instinct is to keep the reticle on or slightly in front of the enemy aircraft. Ignore this instinct, and instead treat your plane like it is a bullet: fly not directly at or slightly in front of it the AI, but instead fly ahead of it as it turns, so that you intercept its flight path instead of falling behind it. This strategy is very effective at getting on the close 6 of an AI that can both fly very fast and turn very hard.
AimingThe best way to aim at an enemy plane is to try to infer what direction the target is moving on your screen (while in the cockpit and not moving the camera), and point your guns some distance away from the plane in that direction of movement—the faster the movement, the more the distance.
A poorly drawn example of this straight from the bowels of MS Paint: