There are two problems with that matchup."But what about av tangrowth with earthquake?"
Tangrowth can eat poop. Intimidate is a thing of beauty.
-1 0 Atk Tangrowth Earthquake vs. 0 HP / 0 Def Manectric-Mega: 116-138 (41.2 - 49.1%) -- guaranteed 3HKO
1) If there are Rocks, Manectric is an unreliable switchin, as Overheat does not OHKO:
-1 0 Atk Tangrowth Earthquake vs. 0 HP / 4 Def Manectric-Mega: 114-136 (40.5 - 48.3%) -- 67.6% chance to 2HKO after Stealth Rock
252 SpA Manectric-Mega Overheat vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 212-250 (52.4 - 61.8%) -- guaranteed 2HKO
2) Even without rocks on Manectric's field, it cannot reliably 2HKO a healthy AV Tangrowth with Overheat. Or, at least, not as reliably as Tangrowth can 2HKO it:
-2 252 SpA Manectric-Mega Overheat vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 106-126 (26.2 - 31.1%) -- guaranteed 4HKO
OVERALL: +0 and -2 252 SpA Manectric-Mega Overheat vs. 252 HP / 252+ SpD Assault Vest Tangrowth: 316-372 (78.2 - 92%) -- 31.3% chance to 2HKO after Stealth Rock (The numbers are not exact as I used a 195 BP Overheat to mimic the halved SpA on the second move, but they have an error margin of less than 10 HP)