109K has all the advantages against a P-38L. The 109 is much faster at all altitudes, accellerates much faster, climbs much better at all altitudes (+1000 fps at SL), and turns much better both with and without flaps. With equal E states and pilot skill the 109 should win every time. My guess is the pilot skills in question are not equal.
A few tips:
If the 38 dives on you, split-S underneath him (use trim if necessary to pull out). He will not be able to follow and make a shot. If he's a noob he might even compress and auger. Extend as much as possible between every split-S to equalize E. Once he has spent his E advantage start building your own E as described below.
If you are equal in alt and E, extend in a low-G zoom climb and use your superior climb to gain altitude on him, and use your superior speed to keep him at arm’s length. You can use the E advantage to either B&Znore him to death or spend the E on saddling up on him.
If you saddle up MIND YOUR SPEED. The P-38 slows down very quickly and you’ll overshoot if not careful. Just throttle back, full rudder, flaps out and kill him. If he forces an overshoot WEP climb above him at the highest angle of attack you dare.
ALWAYS use flaps in a turn fight.
If he is on your six and closing to guns range, NEVER climb. The 38 is much heavier and will out-zoom you. Instead make a rudder-assisted roll inverted and split-S away. Use trim if necessary.
Learn to trim manually. Consider trim to be your high speed flight controls.
