Choose Equipment Wisely
Snoopy has more than just a few types of armaments available to him at the start of any mission. Make sure that you tailor your weapon loadout to best be able to deal with any situation that might arise. Don't just bring along short range weapons, as that will put you at a disadvantage against rocket-equipped enemies. By the same token, make sure you don't skimp on the up-close weapons either, as that will leave you wide open when your enemies close in for the kill.
Run Away
Enemies like to gang up on you in "Flying Ace." Often you will find yourself surrounded on all sides by incoming fighter planes. When this happens, your best bet is to focus on running away very quickly rather than trying to fight them all. Fly as fast as you can in a straight line away from the battle and your enemies will typically give chase, lining up in tight little clusters directly behind you as they do so. This will give you ample opportunity to perform a Quick Turn -- or similar aerial stunt -- to bring your weapons to bear on your pursuers to greater effect.
Use the Terrain
Enemies will have a much easier time following you if they don't have to worry about dodging mountains in the process. This is especially true in multiplayer mode, as human players don't have the same reflexes as computer-controlled opponents. Hug the ground and dodge around obstacles to make it far more difficult for enemies to keep you in their sights. Just make sure you keep an eye on the obstacles as well. It's no good keeping the enemy off your tail if you wind up splattered across the side of a cliff.
Cycle Your Weapons
Snoopy can only have one secondary weapon active at a time, which means that if you stay on one weapon for too long you'll have to deal with reload or recharge times. You can get around this by constantly cycling through your weapons while firing. Each secondary weapon will only get off a few shots at a time, but the volume of fire from all the weapons will more than make up for this. It will also allow you to keep firing, rather than having to constantly wait for weapons to reload or recharge.