Sad or admirable?
There is a smaller park I take my kids to. My kids have been getting frustrated because as soon as we take down the gym and put our 3 pokemon in, this guy shows up within 5 minutes and takes the gym down. I assume he can see the gym from his house. I figured I would put a Blissey in to make it annoying as possible. It was followed by my kid's Snorlax and Alolan Raichu. He showed up like clockwork, and then I realized something...he couldn't defeat my Blissey consistenly at max cp...but he kept trying...I kept my game open at home while I went about my tasks and at 57 minutes, I fed it the 9th golden razzberry...that's when the fun began...I had another hour to feed berries...he took on a different strategy. He would try battling every 10 minutes or so, I assume to let the cp fall a bit or to see if I wasn't paying attention. Anyways, I kept my phone open for the next 4 hours before I had to go to a wedding...here was the result before he finally took it down...

Answers
Want to know a secret? If you don't have anything available to take down a Pokemon at full CP but don't want to risk the owner using a GRB right away, enter and exit the battle a couple of times to lower it's motivation down to a point where you can take it down with ease. Each time you LOSE against a defender, it's motivation goes down ever slightly, which cumulatively can mean taking it down to a CP you can actually beat.
Case in point: I am low on potions and revives all the time and my collection is a perpetual graveyard. Whenever I have to face a gym with a 3200CP Blissey and my reserves just won't cut it (AKA everything from 4500CP to 2500CP is dead or hanging on to dear life by a sliver of health), I apply this pattern until it gets down to the magic 2900 CP and it becomes beatable even by teams of 1400 CP unevolved Rhyhorn.
Of course, there's no point in doing this if said owner is remote feeding already. You'll waste a lot of time which will make it impossible to beat it eleven times within less than ten minutes to beat their berry-feeding limit.
Just a bit of interesting info.