Chansey probably tanks most of his attacks since they're all special. He has immunity to Toxic though and Seismic Toss doesn't have enough PP to actually KO him (80000 HP)
The best bet would be a mix of passive damage and large special defense. A pair of AssVest Regenerator Toxapex with Infestation, maybe? But Sephiroth's damage numbers can scale into the thousands, making that unviable.
>>38064705This seems like the best bet.