You need to make sure that the health bar of your opponent is at the 10% mark, because anything less than that will probably kill it with the first strike of the combo and anything more will leave him with critical energy. Also, you need at least one full meter bar to perform the second part of the combo.
To do the needed 2 hit combo you need to press
(if facing right), and right after the first flame hits your opponent feets, you need to press
to do the Meter Burn thing. I'd recommend a jump distance from your enemy to make sure the first flame actually hits.
If done correctly, you'll see a second big flame on the floor (that's the second hit) that should kill your opponent.
edit: added some extra info