Can I use Mega Charizard Y ex's "Explosion Y" attack if I have two Basic Fire Energy and a Counter Gain tool attached to it if I'm behind on Prizes?
Yes, even though you are only discarding two Energy you can use the attack.
Explosion YCounter Gain



×2

Explosion YDiscard 3 Energy from this Pokémon, and this attack does 280 damage to 1 of your opponent's Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)
Pokémon ex rule
When your Pokémon ex is Knocked Out, your opponent takes 2 Prize cards.
Mega Evolution ex Rule
When your Mega Evolution Pokémon ex is Knocked Out, your opponent takes 3 Prize cards.
Can I use Mega Charizard Y ex's "Explosion Y" attack if I have two Basic Fire Energy and a Counter Gain tool attached to it if I'm behind on Prizes?
Yes, even though you are only discarding two Energy you can use the attack.