Most battles won on a single defender?

It really depends on how stubborn are low-level attacker is when attacking a Blissey. At 1 gym I had my 2997 CP Blissey in there, and an instinct player kept attacking it and timing out. That Blissey got a bunch of victories. Finally decay took over and the player was able to beat it the first time without timing out. So then I gave it a golden berry. (I wouldn't have done that if it had been longer than, like, 11 minutes since I took the gym over in the first place.)

Before teams, in a pinch I'd go and sacrifice useless mons (like articuno or Lv 20 ttar) that might get auto-selected to a second team for an upcoming raid. I'm sure that inflated a few "battles won" stats.