I don`t mean to sound like an ass, but aren't you just proving my point? 2HKO/3HKO on a rocky helmet Garchomp.... maybe I am crazy, but with even the weakest Earthquake(0 atk) you are doing atleast 30%+ correct? I was under the impression that one hit from the Momma does 25% recoil damage, then another from the daughter does another 25%. That is 50% HP on one attack, meaning the next one, Kang is guaranteed to die, ESPECIALLY after an earthquake.
Am I missing something? Isn`t that what a check/counter is? To force something out, or destroy it?
Other people have replied already, but I want to clarify something.
You said Garchomp would switch in on Kangaskhan, use Earthquake and then... eat a Power-Up Punch. That's absurd. Kangaskhan wouldn't use PuP, they would either do 66%+ with Return, or finish you off with Sucker Punch. You may take down Kangaskhan with you, yes, but only if you didn't switch in on Earthquake. In fact,
252+ Atk Parental Bond Kangaskhan Earthquake vs. 252 HP / 0 Def Garchomp: 183-216 (43.5 - 51.4%) -- 7.4% chance to 2HKO
252+ Atk Parental Bond Kangaskhan Earthquake vs. 0 HP / 0 Def Garchomp: 183-216 (51.2 - 60.5%) -- guaranteed 2HKO
You may not even take recoil damage if Garchomp's unlucky enough to switch in on a Earthquake. And would you look at that, if Kangaskhan is at full HP and uses Sucker Punch to finish you off, you will end up losing your Garchomp while she'll be left alive with 3 HP.
You just lost your counter/check to the pokémon you were meant to stop. Out of four possible scenarios (Return->SP, Earthquake->Return, Earthquake->Earthquake, SP->whatever), you lose your counter in three of them and bring down the target in three, two if the target were at full HP. I don't think that's what a counter/check should do, specially when it's Garchomp. Hell, you just had to keep your Garchomp at full HP for the whole match, only to waste it to stop a single pokémon that may still survive your bullshit.