Yache reduces damage by one half, and I was talking about SDYache versions of Garchomp which are by far the most common. Mamoswine is not taking a SD Outrage. It can somewhat deal with Garchomp, weaken it so something else can kill it or finishing it off if it's already weakened, but like you said it's not really a counter for even non Yache versions.
Suspect ladder is the best anyways, you get to see weird stuff like sweeping Jirachis that work great without Garchomp but are setup bait for him otherwise.

