Yeah, you must have been using it wrong. Doing ~70% damage to Lando-T, Rotom-W, and Gliscor with DM is excellent, as you can then 2HKO them with Stone Edge. Or, you could just hit them on the switch with Fire Blast (aside from Rotom-W), which does around 40% to both, and then finish them off with Draco Meteor. And even if they force you out once, big deal, you stripped 2/3 of their physical wall's life and are ready to go to town next time Mega Chomp comes in, or another physical sweeper.
Doing 70% and then switching out would be ok, except with the current EV spread you are doing 70% against Gliscor at max and around 50% against Rotom-W/Lando (you still get the 2HKO on Rotom-W with SE).
With Subject 18s proposed EV spread you just wall break so much better because Draco Meteor actually does what it should. Just some calcs:
252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 0 SpD Hippowdon: 288-340 (68.5 - 80.9%) -- guaranteed 2HKO after Leftovers recovery
0 Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 252+ Def Hippowdon in Sand: 144-169 (34.2 - 40.2%) -- 38.8% chance to 3HKO after Leftovers recovery
Its basically the same against SDef Hippo you do a lot less with DM but the 10%~ more with EQ are enough to 2HKO
252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 0 SpD Gliscor: 280-330 (79 - 93.2%) -- guaranteed 2HKO after Poison Heal
0 Atk Sand Force Mega Garchomp Stone Edge vs. 252 HP / 184+ Def Gliscor in Sand: 96-114 (27.1 - 32.2%) -- possible 5HKO after Poison Heal
252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 4 SpD Landorus-T: 264-312 (69.1 - 81.6%) -- guaranteed 2HKO after Leftovers recovery
-1 0 Atk Sand Force Mega Garchomp Stone Edge vs. 252 HP / 244 Def Landorus-T in Sand: 85-100 (22.2 - 26.1%) -- possible 5HKO after Leftovers recovery
You still need SR and a good damage roll but now at least you have a chance to get past.
252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 0 SpD Slowbro: 265-313 (67.2 - 79.4%) -- guaranteed 2HKO after sandstorm damage and Leftovers recovery
0 Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 252+ Def Slowbro in Sand: 151-178 (38.3 - 45.1%) -- guaranteed 3HKO after sandstorm damage and Leftovers recovery
252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 0+ SpD Rotom-W: 189-223 (62.1 - 73.3%) -- guaranteed 2HKO after sandstorm damage and Leftovers
0 Atk Sand Force Mega Garchomp Stone Edge vs. 252 HP / 252 Def Rotom-W in Sand: 113-133 (37.1 - 43.7%) -- guaranteed 3HKO after sandstorm damage and Leftovers recovery
-2 252+ SpA Mega Garchomp Draco Meteor vs. 252 HP / 0+ SpD Rotom-W: 94-112 (30.9 - 36.8%) -- 74.7% chance to 3HKO after sandstorm damage and Leftovers recovery
You could also do this with the current spread but with SR you don't need to gamble with SE now (you just 2HKO with DM)
252+ SpA Mega Garchomp Fire Blast vs. 252 HP / 252+ SpD Ferrothorn: 324-384 (92 - 109%) -- 87.5% chance to OHKO after Stealth Rock
252+ SpA Mega Garchomp Fire Blast vs. 252 HP / 0 SpD Skarmory: 334-394 (100 - 117.9%) -- guaranteed OHKO
252+ SpA Mega Garchomp Fire Blast vs. 4 HP / 0 SpD Excadrill: 354-418 (97.7 - 115.4%) -- guaranteed OHKO after Stealth Rock (fuck you Air Balloon)
0 Atk Sand Force Mega Garchomp Earthquake vs. 252 HP / 252+ Def Eviolite Chansey in Sand: 295-348 (41.9 - 49.4%) -- guaranteed 2HKO after Stealth Rock and sandstorm damage
The few points you get by investing into attack rarely ever matter because 90% of the time they are overkill, you miss the 80% chance to 1HKO Azumarill with SR and Sand and the OHKO on Goodra with EQ.