Chansey and Blissey are pretty much impossible to kill with the usual Swagger + Substitute + Foul Play + Thunder Wave Klefki, for what it's worth. Chansey is probably the sole reason why I'm able to reliably avoid a hax defeat when someone tries to throw a swagger Klefki against my uber team. Also note that Chansey and Blissey can do realistic damage back with Seismic Toss on those turns when it isn't immobilized.
Note that having 0 IVs or 31 IVs in Attack on Chansey makes a dramatic difference when dealing with a maximum strength Foul Play:
+6 0 Atk Klefki Foul Play vs. 4 HP / 252 Def Eviolite Chansey: 22-27 (3.4 - 4.2%) -- possibly the worst move ever (0 Atk IVs on Chansey)
+6 0 Atk Klefki Foul Play vs. 4 HP / 252 Def Eviolite Chansey: 69-82 (10.7 - 12.7%) -- possible 8HKO (31 Atk IVs on Chansey)
+2 0 Atk Klefki Foul Play vs. 252 HP / 252+ Def Eviolite Chansey: 11-13 (1.5 - 1.8%) -- possibly the worst move ever (also 0 Atk)
Blissey arguably does even better in a practical sense. Because of leftovers healing 6.25% damage per turn, at +2, confusion and Foul Play won't even break past Blissey's passive healing. At +6, though Blissey technically takes more Foul Play damage than Chansey, with Leftovers it actually ends up with more net health per turn despite taking more damage per strike:
(all 3 examples assume 0 Atk IVs, as +6 Foul play does something like 20% with 31 IVs.)
0 Atk Klefki Foul Play vs. 4 HP / 252 Def Blissey: 13-16 (1.9 - 2.4%) -- possibly the worst move ever
+2 0 Atk Klefki Foul Play vs. 6 HP / 252 Def Blissey: 26-31 (3.9 - 4.7%) -- possibly the worst move ever
+6 0 Atk Klefki Foul Play vs. 4 HP / 252 Def Blissey: 51-61 (7.8 - 9.3%) -- possibly the worst move ever
In both cases, you have to be stun-locked for about 20 turns in a row to be unable to get a Softboiled in (and if you're getting stunned that much, Klefki is in real danger of losing a PP stall war). Also note that Ditto gains absolutely nothing from transforming into a +6 Atk Chansey/Blissey either.
The nice part about Chansey/Blissey is that they're actually useful in the metagame and will pull their weight even if you don't come across a Prankster Swagger pokemon. It's still possible for the non-Klefki versions to annoy them somewhat with Taunt (though they give up Thunder Wave to do this), but they're still more reliable than the majority of Pranksterswag counters out there.