I don't know how well this will work in the GS Cup rules, but Rotom-Wash can do both of those things and has bulk to spare. It does well against Primal Kyogre, walls Primal Groudon, and resists Mega Rayquaza's Dragon Ascent. Watch for Xerneas, though, who can tear Rotom to shreds after a Geomancy and doesn't give a crap about Hydro Pump or Thunderbolt.
For partners, I suggest Mega Rayquaza, whose Delta Stream will cancel Groudon's Desolate Land and let Rotom OHKO with Hydro Pump.
Alternatively you could try Giratina, who gets Will-o-Wisp plus Phantom Force to break Protect, or Earth Power to level Primal Groudon. It doesn't get Reflect, unfortunately.