Oh wow. Because you have annoyed me so, I'm going to critique everything! >:]
"Once during your turn, you may flip a coin."
I need to know exactly when in my turn I can use the Ability. I assume 'before your attack', so there should be the
(before your attack) after 'turn'.
"...you may flip a coin. If that coin is heads, flip another coin. If that coin is heads..."
This is a complicated way to flip two coins when you could have said "...you may flip 2 coins. If both of them are heads...".
"During your next turn, that Pokémon has 20 more HP."
This needs a 'Then,' at the beginning, so I am a least a little bit less confused and so you are periodically accurate.
"If you discard it, flip a coin."
You are flipping 'another coin', remember.
"If heads, shuffle all energy cards back into your deck."
Energy cards from where? Also, energy should be Energy.
"During your next turn, flip a coin."
Another 'Then,' goes at the beginning here. And you're flipping 'another' coin. :3
"If heads, draw a prize card. If tails, your opponent draws a prize card."
'prize' should be captilised as 'Prize'.
"Either way, this Pokémon takes 20 damage."
I do hope you know that you only take 20 damage if you had to flip a coin for a Prize card...
"Your opponent reveals his or her hand. Discard the top card of your opponent's deck for each card in his or her hand. Then, your opponent puts his or her hand to the side, and it cannot be altered or touched in any way until the end of his or her next turn."
Firstly, this is overpowered cause you lock your opponent into dead draw immediately. Secondly, there is actually an amazing Empoleon Lv.X that does a very similar thing to this. Using that as reference, the attack would be worded something like this:
"Your opponent reveals his or her hand. Discard the top card of your opponent's deck for each card in his or her hand. Then, your opponent puts his or her hand face down next to their Active Pokémon. (These cards are not in play or in your opponent’s hand.) At the end of your opponent’s next turn, return those cards to your opponent’s hand."