Garchomp hasn't been much of an issue lately since I've been using my Hail team a lot, but Mamoswine I haven't seen much, period. Everyone on my team can hit it SE and Wailren can sponge attacks not named Stone Edge, though.
I find that Staraptor @ Choice Band can KO both despite weakness to Ice.
130 Base Attack. Ice and Ground resisted by one Pokemon. Add Choice Band to that. People should also stop assuming it'll use Ice Shard instead of Ice Fang. Celebi takes hefty damage from A CB Ice Fang. When you consider that most Celebis only run enough speed to outspeed Jolly Tyranitar and that Mamoswine has a max speed of 259 you start to realize it's likely to outrun Celebi. Besides, the main problem is switching something in to Mamoswine. Cresselia and Bronzong is just about the only things safe when considering a switch in.Best Garchomp counter: Ice Beam (It's finding the best user that's hard)
For Mamoswine: Just hit the damn thing till it dies. It's slow and has weak defenses, i really don't see how it is as threatening as people make it out to be.
Now, as to the best user of ice beam (Or any ice attack for that matter) you'd want a bulky pokemon, porbably some kind of bulky water, to take it on. Swampert comes to mind, and so does Milotic.
As for Mamoswine, Cresselia puts the beat down on it (Especially calm mind variants), and Bronzong lols at Mamoswine's ice shard and hurts it back with STAB super-effective Gyro Balls (though they will be quite weak due to Mamoswine being slow)
Toxic Spikes
Garchomp makes a semi-efficient mamo counter actually.