I prestiged a friendly gym yesterday that had 2 Chanseys at the bottom (800 CP and 900 CP). Generally, I don't try to match the CP of the Chanseys for optimal prestiging, I just match to the mons behind them.
I could get through both with an Arcanine with 1900 CP (Bite / Fire Blast) by dodging the charge moves. Rest of the mons in the gym were over 2000 CP, so I still got good prestige once I got past the Chanseys. I used 6 mons around 1900 CP and got about 3500 prestige after clearing the gym (was Level 9). Took me 3 swings but I got it up to Level 10 eventually.
Those Chanseys sure are annoying though (it's not really a fight, it's like pounding sand). Really not looking forward to Blissey...