The only planes that I think will lose to an A20, assuming equal skilled pilots are:
Me-410
190F8
190A8 (maybe)
Ta-152 (on the deck, TnB)
P40s
P39s
In a DA environment, 1v1 merge, the other plane needs to be able to turn/roll better than the A20, to bleed its speed, and then have the acceleration required to start an uphill battle, exceptions being the extreme turny birds (A6M, Hurri, Brew) who can just flat turn and beat it. If the plane cannot force the A20 on the defensive by hard maneuvers, then it will not win. For example, the Ta152 out climbs theA20 in the charts, but in a 1v1 fight, the A20 can turn with it, forcing the 152 on the defensive. The 152 cannot use its climbing power because it's too busy trying to keep the A20 of its tail. The A20 has the inertia to go in a shallow dive then zoom up, giving it plenty of time to kill the other plane.