Human fighters in this instance are six WWII Spitfire Vs and one B-17 Bomber modified with two extra turrets. Each of these planes has been refitted with shields, which can protect from 2 shots on the fighters and 12 on the bomber. The bomber does NOT drop bombs, but instead has what is basically an energy grappling hook turret on the bottom which can be used to tow or be towed or stick fighters to each other. Each of these planes have a gravity engine capable of speeds of up to 100 m/s(~220 mph) for short bursts, and a maximum sustainable speed of 70 m/s(~150 mph). The gravity engines can also be used as a short range offensive weapon, temporarily turning the planes to propeller propulsion and creating a strong gravitational anomaly for a few seconds, capable of diverting enemy fire, quick maneuvers, and if done right even imploding enemy craft. Fighters are armed with four machine guns, bomber has two top turrets, two bottom turrets, four side guns(two left two right), and one rear turret, all of which are machine guns.
The enemy craft are alien spacecraft armed with energy weapons and have shields designed for protection against energy weapons, which is one advantage the humans have. Since the fighters and bomber use physical projectiles, the shields quickly fall but a lucky shot is required to disable a key system or kill the pilot, or they could swish cheese it but that takes a lot of ammunition. Enemy fighters can travel at speeds up to 400 m/s(~900 mph).
So even though the humans can easily take down enemy shields, a lucky hit or missile is needed to completely disable the enemy craft. Since the former is rare, and the latter is in short supply, they get stuck in extremely long dogfights which can have very bad outcomes for humans. And then since the aliens can go about 6 times faster than them, the humans don't really get to decide the battlefield. Are there any ways I can have the human ships stand a chance against these aliens, preferably without changing the human or alien ships?
Reason why military fighters aren't zooming around shooting everything-
Since the aliens first started attacking, military fighters were the first thing to go and were quickly shot. These WWII planes were donated by some crazy rich old guy and crammed with whatever alien tech would fit and some machine guns, so they're some of the last airborne units around.