Mega-Gar is a 50/50 proposition. Attack and you risk getting Sucker Punched, Sub and you risk getting Fire Punched. Not to mention gar probably doesn't like eating the second part of a Sucker Punch even if it guesses correctly to begin with.
People forget that Mega-Khan can always run Crunch and straight-up own all Ghosts bar Evolite Duskclops. A lot of criticism towards Mega-Khan is not towards Khan itself... but is really criticism towards the guessing games that Sucker Punch entails. Running Crunch removes the "guessing game" aspect and hits Ghosts just as hard.
Khan can usually get +1 easily, although the +2 is difficult because most people switch into ghosts when dealing with Khan. So I'm only going to do dmg calculations assuming a Scrappy +1 Power-up Punch on the switch. I'm going to run Choice Band to "simulate" the double-strike effect of Parental Bond.
Don't even need +1 here:
252 Atk Choice Band Kangaskhan Crunch vs. 4 HP / 0 Def Gengar: 384-454 (146.56 - 173.28%) -- guaranteed OHKO
252 Atk Choice Band Kangaskhan Earthquake vs. 68 HP / 0 Def Chandelure: 348-410 (125.17 - 147.48%) -- guaranteed OHKO
The +1s hit the more defensive ghosts:
+1 252 Atk Choice Band Kangaskhan Crunch vs. 248 HP / 252+ Def Jellicent: 344-406 (85.35 - 100.74%) -- 6.25% chance to OHKO
+1 252 Atk Choice Band Kangaskhan Earthquake vs. 252 HP / 0 Def (Aegislash Custom): 336-396 (103.7 - 122.22%) -- guaranteed OHKO
+1 252 Atk Choice Band Kangaskhan Crunch vs. 252 HP / 252+ Def (Trevenant): 328-386 (87.7 - 103.2%) -- 25% chance to OHKO
+1 252 Atk Choice Band Kangaskhan Crunch vs. 248 HP / 252+ Def Cofagrigus: 212-250 (66.45 - 78.36%) -- guaranteed 2HKO
+1 252 Atk Choice Band Kangaskhan Crunch vs. 252 HP / 252+ Def Eviolite Dusclops: 154-182 (54.22 - 64.08%) -- guaranteed 2HKO
+1 252 Atk Choice Band Kangaskhan Crunch vs. 252 HP / 252+ Def (Gourgeist): 238-282 (63.63 - 75.4%) -- guaranteed 2HKO
So we have our list of counters to PuP / Crunch set. Cofagrigus (I dunno how Mummy works, but I'm assuming that after PuP, Khanga looses Scrappy. THEN he mega-transforms, gains Parental Bond, strike twice, and then loses Parental Bond)
, Dusclops, and Gourgeist. The other ghosts don't actually stand a chance. The PuP strategy is a bit of a guessing game, because Khanga stays at 80 base speed for the 2nd turn. Nonetheless, it locks in kills vs most Ghosts, and then sets you up for a very good sweep.
--------------------------------------
A strange note: it is actually better to switch in a Steel Type / Rock Type and take the Super-Effective Power-up Punch repeatedly, rather than take the unresisted Return, SE Crunch, or SE Earthquake.
Therefore, Ferrothorn can take non-Fire Punch variants, as can Skarmory. Skarmory can't do jack **** outside of phaze however.