As Giratina’s HP are already so high, I think going Max Def instead of Max HP has a higher impact. However, I recommend going for Max speed anyway. It’s too bad you have a Modest one, Shadow Force and Phantom Force are immediate under new moon, giving you an amazing STAB.
I also recommend Rest or Pain Split for Spiritomb. Sacrificing your only weather setter is a very bold move.
I only have 1 set-up sweeper so I don’t need Spiritomb to last forever, he usually uses will-o-wisp/knock off on the first turn then switches out and memento’s the next time he comes in to allow scyther to set-up easily and sweep whatever’s left. My other members are strong enough to hold their own without the weather, the weather is just a bonus so losing Spiritomb isn’t the biggest loss for the team.