The basic point of this setup is, in an environment without the Pokemon Clause, to have choice scarf Greninjas whose only purpose is to use mat block one turn and switch out the next. This coupled with a magic bounce mon such as mega Diancie should render the team impervious to non-priority, non-protection breaking attacks.
One would have two Greninjas on each side of the Diancie. One would use mat block, and the other would switch out for a third greninja. The one that just switched in uses mat block on the next turn while the one that can't use mat block switches out, and so forth. Meanwhile, Mega Diancie is free to set up and/or sweep.
The holes in this that I see are:
Mold breaker (only applies to status moves)
Feint/Shadow Force/Phantom Force (overall weak moves)
Priority (by far the largest concern)
Is this a sound strategy?