My Shaymin was in doubles with my Shiny Defensive Mew(omg so rare :o ) so I'm like, well that sux m8. Any reasons how it lived (Gre was Life Orb)
First of all, I'm guessing your shaymin was land form - since his defense is much higher than sky form's. Here are some other possibilities:
1) Your shaymin's defense/sp. defense could've been maxed out or have a nature that boosts it.
2) Greninja could've had zero sp. attack boost or a nature that lowers it
3) Your team could've raised it's defense/sp. defense or use a move like "light screen"
4) You could've lowered your opponents sp. attack.
Maybe you were just lucky, but who knows. Hope this helped! :)