That card would be broken. You could choose not to do damage every turn and deck out your opponent if he or she doesn't have any or has run out of cards that would switch Wailord Lv. X with somebody else.
What would really annoy Gallade is a Pokémon whose maximum HP is 50 but is either immune to high-damage attacks (like the Jungle Mr. Mime) or reduces them to a ridiculous degree (like one of the Shuckles, forgot which). Of course, that Pokémon would still be vulnerable to a Gardevoir's Bring Down...unless it was also immune to other Pokémon Lv. X's. I can see it now...
Shedinja Lv. X
50 HP
Psychic
Poké-Body - Wonder Guard
If the Defending Pokémon attacks Shedinja Lv. X and is a Pokémon Lv. X or does 20 or more damage (before applying Weakness and Resistance), prevent that damage.
[P][P] Shadow Sneak 40
Flip a coin. If heads, prevent all damage done to Shedinja Lv. X during your opponent's next turn.
Weakness: Darkness x2
Resistance: Fighting -10
Retreat Cost: 1
This card would shut down G&G all by itself.