Trappers and Trick have been mentioned.
Gengar V Chansey is kind of a stalemate... It's possible for Chansey to PP stall depending on how much PP both Pokemon have left on their moves/what moves they're running. I'll still make a mention, though.
Thanks for the suggestions :)
I guess if Gengar gets (very) lucky with Focus Blast, or uses Sludge Bomb and poisons, Gengar can come out on top - and typically Gengar would probably be more than happy to get Chansey out of the way for the rest of its team.
252 SpA Gengar Focus Blast vs. -1 252 HP / 4 SpD Eviolite Chansey: 252-298 (35.7 - 42.3%) -- guaranteed 3HKO after poison damage (but who runs Sludge Bomb, Focus Blast and Substitute - and if Gengar is Life Orb then it's screwed).
However, Gengar with Pain Split can really really screw up Chansey, as Pain Split does twice as much as any of Gengar's attacks (and I am a fan of SubSplit Gengar myself). Also, there are certain fun Gengar sets that can completely screw up Chansey, such as Shadow Ball/Substitute/Disable/Taunt. I myself love this set as well and it does other things apart from completely screw up Chansey/Blissey/everything else lol. but i actually really like Chansey, unlike Blissey (stupid fat bitch lol).
There is also a damage percentage point that separates Chansey from Blissey. I can't remember exactly what it is, but if i recall, Blissey takes moves that do 33% or less (not completely sure about the exact number, i'll go hunting for it and tell you later) better than Chansey due to Blissey having Leftovers recovery (and Blissey can Wish+Protect and get Leftovers recovery from that too). Blissey can also do things like run Ice Beam/Flamethrower/etc to clip Scizor/Gliscor with on the switchin, whereas Chansey can't do anything except Seismic Toss - I think it's worth mentioning the differences between the two at some point.
However, Chansey's ridiculous bulk is apparent when you start to play with the damage calculator:
+4 252 SpA Thundurus Thunderbolt vs. 252 HP / 4 SpD Eviolite Chansey: 273-322 (38.7 - 45.7%) -- guaranteed 3HKO
as compared to
+4 252 SpA Thundurus Thunderbolt vs. 252 HP / 4 SpD Blissey: 330-388 (46.2 - 54.3%) -- 5.9% chance to 2HKO after Leftovers recovery
There is also an optimised spread for Chansey to take hits as best it can on both sides of the spectrum, and it's not 252HP/252 Def Bold or 252 Def/252 SpDef Calm I don't think. Near max Defensive investment (maybe not 252, maybe 240 to hit an eviolite jump point) is definitely a must as it almost triples Chansey's physical bulk - I'll track down the spread in question if i can (or maybe a QC member knows it, it mightj have been something weird like 112 HP/244 Def+/rest in SpDef, i really can't remember - this was a huge discussion in the UnderUsed room back when Chansey was in UU)
sorry for the huge paragraph of stuff, great job on this analysis though, like i said i always liked Chansey and have always disliked Blissey, so I'm glad that Chansey is better this generation :)
p.s. sorry i missed the Trick/trapper mentions, Gothitelle is definitely the best way of removing Chansey as a defensive threat though :)