Weclome, people. I've heard alot of trouble about people countering Garchomp recently, so I'm here to post the ultimate counter.
Donphan w/Haban Berry
*Indepth info about EV spread later*
Adamant Nature
Ice Shard
Earthquake
Rest/ Roar/ Other Support Move
Rest/ Roar/ Other Support Move
So here it is, bulky enough to take Outrage and strong enough to retaliate with Ice Shard. Haban Berry works to lower the damage of said Outrage, or Draco Meteor from Chainchomp or Dragon Claw from SD Chomp.
Earthquake is there for power and also offers good coverage alongside Ice Shard.
The last 2 spots are really not important, although I would suggest Rest to heal off the major dent you're about to take.
Now for the EVs-
Adamant Donphan w/ 148 Atk EVs can 2HKO Standard 12HP Garchomp, with SR down it will do the same through Leftovers, if you want to be safe and take out Garchomp without SR you will need 224 EVs, I wouldn't worry about that.
To 2HKO taking into account Yache Berry you will need 252 Atk EVs and SR set up to have a chance, but without Choice band there is no way to guarantee it. With Choice Band you need 24 EVs if Adamant, 136 Impish.
On the Defensive side-
Thanks to Haban Berry Donphan takes only a 3HKO from Outrage with a spread of 252HP/110Def, by which time you can have him 2HKO'd, unless he has Yache berry in which case you'll have to finish it off with something else, but you'll make a tasty dent either way.
Haban Berry once again helps against ChainChomp Draco Meteor as, given that it hits for maximum damage each time, the same 252HP/110 Def Donphan is only just 2HKO, so 4 Sp Def Evs should be added just for that. Thanks to life orb recoil Garchomp cannot resist the 2HKO.
With SD Chomp, lets assume you switch in on the SD.
First turn, you Ice Shard, Yache Berry kicks in and Chomp takes 90 damage minimum. He retaliates with a +2 Dragon Claw and you take 249 damage maximum, due to Haban Berry. Next Ice Shard again and you do a minimum of 180 damage and take another Dragon Claw for 298 damage maximum, infact even at minimum damage both times you are now Ko'd. This is the most troublesome form, to guaratee that KO you'll need 3 layers of spikes down, which is very situational. Still, you've just left him on 25% HP without a Yache Berry so he isn't that hard to finsih off.
To summarize-
Donphan w/Haban Berry
252 HP/ 148 Atk/ 106 Def/ 4 Sp Def
Adamant Nature
Ice Shard
Earthquake
Rest/ Roar/ Other Support Move
Rest/ Roar/ Other Support Move
Anti- Chomp Donphan =D.
Please "Rate" away, any and all (mature) criticism is welcomed.

Donphan w/Haban Berry
*Indepth info about EV spread later*
Adamant Nature
Ice Shard
Earthquake
Rest/ Roar/ Other Support Move
Rest/ Roar/ Other Support Move
So here it is, bulky enough to take Outrage and strong enough to retaliate with Ice Shard. Haban Berry works to lower the damage of said Outrage, or Draco Meteor from Chainchomp or Dragon Claw from SD Chomp.
Earthquake is there for power and also offers good coverage alongside Ice Shard.
The last 2 spots are really not important, although I would suggest Rest to heal off the major dent you're about to take.
Now for the EVs-
Adamant Donphan w/ 148 Atk EVs can 2HKO Standard 12HP Garchomp, with SR down it will do the same through Leftovers, if you want to be safe and take out Garchomp without SR you will need 224 EVs, I wouldn't worry about that.
To 2HKO taking into account Yache Berry you will need 252 Atk EVs and SR set up to have a chance, but without Choice band there is no way to guarantee it. With Choice Band you need 24 EVs if Adamant, 136 Impish.
On the Defensive side-
Thanks to Haban Berry Donphan takes only a 3HKO from Outrage with a spread of 252HP/110Def, by which time you can have him 2HKO'd, unless he has Yache berry in which case you'll have to finish it off with something else, but you'll make a tasty dent either way.
Haban Berry once again helps against ChainChomp Draco Meteor as, given that it hits for maximum damage each time, the same 252HP/110 Def Donphan is only just 2HKO, so 4 Sp Def Evs should be added just for that. Thanks to life orb recoil Garchomp cannot resist the 2HKO.
With SD Chomp, lets assume you switch in on the SD.
First turn, you Ice Shard, Yache Berry kicks in and Chomp takes 90 damage minimum. He retaliates with a +2 Dragon Claw and you take 249 damage maximum, due to Haban Berry. Next Ice Shard again and you do a minimum of 180 damage and take another Dragon Claw for 298 damage maximum, infact even at minimum damage both times you are now Ko'd. This is the most troublesome form, to guaratee that KO you'll need 3 layers of spikes down, which is very situational. Still, you've just left him on 25% HP without a Yache Berry so he isn't that hard to finsih off.
To summarize-

Donphan w/Haban Berry
252 HP/ 148 Atk/ 106 Def/ 4 Sp Def
Adamant Nature
Ice Shard
Earthquake
Rest/ Roar/ Other Support Move
Rest/ Roar/ Other Support Move
Anti- Chomp Donphan =D.
Please "Rate" away, any and all (mature) criticism is welcomed.