this thing is the monster of my nightmares how do I kill it and nip in the bud (as soon as possible)
Fighting-types: Physical Fighting-types such as Mega Medicham, Mega Gallade, Mega Lopunny, and Mega Heracross all threaten to OHKO Chansey or do massive amount of damage with their respective Fighting-type STAB moves. Keldeo is also worth mentioning because Secret Sword targets Chansey's physical defense stat.
Physical Wallbreakers: While Chansey's poor defense stat is remedied with heavy investment and Eviolite, strong physical breakers can still muscle through it quite effectively. Some examples include Landorus-T, Mega Mawile, Tapu Bulu, Choice Band Tyranitar, Swords Dance Garchomp, and Kyurem-B.
Losing Eviolite: The removal of Eviolite is a big problem for Chansey, severely diminishing its bulk, which makes it far tougher for it to handle many of the offensive threats that it would usually be able to deal with. Knock Off users such as Clefable, Tornadus-T, and Alomomola can lure Chansey in and remove its Eviolite. Trick users such as Latios, Gengar, and Hoopa-U can be even more devastating, as not only do they remove Chansey's Eviolite, but they also lock it into a single move, which heavily limits its effectiveness.
Taunt Users: Seismic Toss is Chansey's only attacking move, meaning that most of its moveset is useless if it gets Taunted, making it near impossible for Chansey to perform its role effectively.
source: checks and counters section https://www.smogon.com/dex/sm/pokemon/chansey/
