.. any combination of a plane that turns better than mine, and a plane that is way faster than mine.
A Spit16 or a La-7 is just another meatbag when alone, but the combination of these two in the area makes your world really, really miserable, especially when you are outnumbered.
Some more filthy, disgusting team ups would be:
La-7 + N1K2 : classic AH dweeb combo
La-7 + Ki-84 : 'you're owned' combo
F4U-1C + Seafire : 'it came from the ocean' combo
20k P-38 + 15k Typhoon + deck Spitfire : the "Akak-Damned Group" vulchpick collaboration