Fantastic emergency check to just about anything physical. This guy can even counter Swords Dance Garchomp in a pinch: +2 EQ doesn't OHKO 252/0 MegaAggron and +0 EQ deals about 36%. Aggron can retaliate with Ice Punch
In fact, Swords Dance Garchomp needs a Life Orb to OHKO a completely uninvested, non-Defense-boosting nature, full-health Mega Aggron with a +2 Earthquake
+2 252 Atk Garchomp Earthquake vs. 0 HP / 0 Def Filter Mega Aggron: 234-276 (83.27 - 98.22%) -- guaranteed 2HKO
With an Impish nature, Mega Aggron has a (Very small) chance of surviving said Earthquake at full health. Remember - NO INVESTMENT at all, just a positive nature and Garchomp needs both Life Orb and SR to OHKO it
+2 252 Atk Life Orb Garchomp Earthquake vs. 0 HP / 0+ Def Filter Mega Aggron: 277-328 (98.57 - 116.72%) -- 93.75% chance to OHKO
Add some investment and it starts needing residual damage. Forget about OHKOing it if it's running Yache
Keeping with the examples, now adding 3 Attacks Gengar's Focus Blast:
+2 252 Atk Garchomp Earthquake vs. 252 HP / 0 Def Filter Mega Aggron: 234-276 (68.02 - 80.23%) -- guaranteed 2HKO
252 SpA Life Orb Gengar Focus Blast vs. 252 HP / 252+ SpD Filter Mega Aggron: 213-252 (61.91 - 73.25%) -- guaranteed 2HKO
252 SpA Life Orb Mega Gengar Focus Blast vs. 252 HP / 252+ SpD Filter Mega Aggron: 259-306 (75.29 - 88.95%) -- guaranteed 2HKO after Stealth Rock
+2 252 Atk Garchomp Earthquake vs. 252 HP / 252 Def Filter Mega Aggron: 207-244 (60.17 - 70.93%) -- guaranteed 2HKO
252 SpA Life Orb Gengar Focus Blast vs. 252 HP / 0+ SpD Filter Mega Aggron: 281-331 (81.68 - 96.22%) -- guaranteed 2HKO (18.75% chance to OHKO after Stealth Rock)
252 SpA Life Orb Mega Gengar Focus Blast vs. 252 HP / 252 SpD Filter Mega Aggron: 285-335 (82.84 - 97.38%) -- 25% chance to OHKO after Stealth Rock
+2 252 Atk Garchomp Earthquake vs. 252 HP / 252+ Def Filter Mega Aggron: 190-225 (55.23 - 65.4%) -- guaranteed 2HKO
252 SpA Life Orb Gengar Focus Blast vs. 252 HP / 0 SpD Filter Mega Aggron: 308-363 (89.53 - 105.52%) -- 31.25% chance to OHKO (68.75% chance to OHKO after Stealth Rock)
A Careful nature seems to be enough to significantly reduce KO chances from powerful, special coverage moves while still being an immovable object for physical attacks