So, I'm trying to find more information on Mat Block for Greninja, but none of the database sites have the details on it. The way it is worded, it is theoretically broken, being able to block all attack moves for it's whole team each turn.
"Using a pulled-up mat as a shield, the user protects itself and its allies from damaging moves. This does not stop status moves."
However, in practice, it seems to only work once in a battle, which really isn't very useful. Despite it's description not saying so, it will fail if you try to use it again, every time you attempt to use it.