Why? It's not Endrevving.
A Timid Scarfcham easily OHKOs Garchomp.
Adamant Scarfchomp outrage easily OHKO's medicham.
Swampert with avalance can beat them.
Thats what I use.
Specsmence Draco Meteor can OHKO and all garchomp has to do is Swords Dance with a life orb and it's game over for pert.
I find it strange no one has mentioned a bulky Starmie. As far as I know (and I could be wrong, never used one before), it outspeeds both and should be able to OHKO both with Ice Beam. Also allows for RSing and Recovering.
Starmie frowns at Draco Meteor and a really bulky set will still get 2hko'd by scarfchomp before it can ice beam back.
Also Dusknoir doesn't take draco meteors well and Swords dance garchomp w/ life orb wrecks it.
The only pokemon that can really counter both of these at the same time is Cresselia. However, after numerous draco meteors, stealth rock switch ins, etc, Cresselia will be forced to recover pretty much every time it wants to switch in against a specsmence. This leaves plenty of time to get a free ttar switch in or whatever else you want to use to set up. Since a lot of teams that have garchomp on them also run ttar, most cresselia I've seen lately have been rest talking. This means often times it will become a luck game to see if you get that 1 in 3 chance of sleep talking ice beam. (not to mention random sand veil hax as well)
Blissey does a nice job at walling specsmence, but a lot of players have been using the mixmence with brick break lately so she might take a chunk of damage on the first or second switch in. However, even if the player chooses draco meteor, you can tell it's mixed by the life orb recoil. This basically leaves the rest of the game up to prediction. Other Pokemon mentioned in the analysis for both garchomp and salamence do a decent job at countering these huge threats as well. Overall, these two pokemon can be a huge pain at times. Prediction is pretty key when trying to guess what set and then what move they will make.