This is what you have above:
252SpAtk +6 Flash Fire Chandelure (Neutral) Hidden Power (Fighting) vs 252HP/252SpDef Leftovers Thick Fat Munchlax (+SpDef): 111% - 131% (528 - 622 HP). Guaranteed OHKO.
The only way to get this is if they are both on the same level.
Below are all calcs that ran the same levels and EV setup as above. Obviously, I don't know the exact EVs, but hey. Each calc has a different nature that either raises, lowers, or does not effect SpDef on Munchlax.
Negative
252SpAtk +6 Flash Fire lvl 70 Chandelure (Neutral) Hidden Power (Fighting) vs 252HP/252SpDef Leftovers Thick Fat lvl 95 Munchlax (-SpDef): 76% - 89% (342 - 404 HP). Guaranteed 2HKO.
Neutral
252SpAtk +6 Flash Fire lvl 70 Chandelure (Neutral) Hidden Power (Fighting) vs 252HP/252SpDef Leftovers Thick Fat lvl 95 Munchlax (Neutral): 68% - 80% (306 - 362 HP). Guaranteed 2HKO.
Positive
252SpAtk +6 Flash Fire lvl 70 Chandelure (Neutral) Hidden Power (Fighting) vs 252HP/252SpDef Leftovers Thick Fat lvl 95 Munchlax (+SpDef): 62% - 73% (280 - 330 HP). Guaranteed 2HKO.
No matter what, it is a 2HKO. You need to remember that Munchlax has awesome HP and nice SpDef.
