Well, I used my 2100CP Machamp to train my own Chansey with 1020CP. My Machamp was killed in the battle. My Chansey has Pound/hyperbeam moveset. I dodged the first hyperbeam but didn't dodge the rest because I was afraid of running out time.
Nearby, we have two gyms next to each other. We took both gym. Then another player (in different color) took down one gym. I could see that he is at level 22 after he put his dragnite in. The other gym had my Chansey. He spent almost an hour to attack the gym and eventually gave up. I guess without his dragnite, he just could not defeat my Chansey.