La-7 is just a ridiculously good tool at low level. Just stick to the vertical plane for maneuvering and you will rule.
Also, deflection shooting is not so easy because of the view over the nose. So it helps to "saddle up" (get on dead six).
But it is not always wise to maneuver with the bandit to saddle up (need to keep enough energy to escape in case more/higher bandits show up), so you need to make some high-deflection snapshots. The poor deflection-shooting view in the La-7 means you need to minimize range and do some tricks to maximize visibility of the bandit.
What you have to do is maneuver so that he crosses your gunsight from left-to-right or vice-versa, and he crosses at very close range. This means you have to guess at his future flight path, but most MA denziens tend to be predictable. Usually they see you coming in fast somewhere behind their wing-line, and make at least a quarter-turn blacked out.
If you see this coming, do a high yo-yo into his turn, and try to come in at a 90-degree angle to his flight path, wings-level with your pipper just below and ahead of him. When the time is right (it takes some practice), pull up and "have a squirt" as some RAF pilots used to say.

If you really timed it right, you will almost collide with him, leaving a trail of hit flashes from nose to tail, passing just behind his tail as you switch views to watch his smoking carcass spin out of control. It's a very high deflection shot, and he might be pulling a lot of gees, but by predicting his flight path and making it a very short-range shot, you take those factors out of the equation. In essence you lead him with your plane so you don't have to lead him with your bullets.
If he's not fatally damaged, or he changed his maneuver, just put your nose up and zoom, and let the manly ASh-82FN pull you skyward, then try again. As long as he keeps turning hard, and as long as you keep replenishing your energy with zooms, your energy advantage over him will grow and grow. The longer this goes on, the more chances you get to shoot him, and the more likely it is that he will screw up and give you a dead-six shot.
Now to deal directly with PRZ's question: If the guy in the Yak is better than you, forgetaboutit - the difference in the planes is not big enough to overcome a big difference in the pilots. Assuming both pilots are equal, and he is behind you with equal energy, use your speed to exit and come back later. But if he is not on your tail, and you have somewhere near equal energy, you should be able to use your climbing and top speed advantages to beat his turning ability. Use vertical maneuvers and gunnery techniques like I described above, and you should be OK at low levels. Up higher, the La-7's performance drops off a little quicker than the Yak-9U, so you will need to be a better pilot or have a little more energy.
PS Solution space? NERD!!!
[This message has been edited by funked (edited 03-21-2001).]