Protect or Slack Off would do wonders. You could also change Slow Start so that it only drops Speed and not Attack. That would help too.
Letting it learn Skill Swap would make it really freaking annoying.
If you want to cut out Slow Start all together, switch it out for Inner Focus, Clear Body so it can be like the other Regis, Anticipation, Overcoat, Unnerve, and Sturdy, since it's so bulky almost nothing will OHKO it unboosted. All of those abilities are niche abilities that would only give it a minor advantage. Most of them would do nothing at all in 90% of games. You want it to be good, like ubers good with an ability that makes sense?
Weak Armor, It has the bulk to pull this off
Regenerator
Dry Skin so it can be the perfect counter to Kyogre
One last idea:
Crush Grip, it's signature move, sucks donkey balls. Change that from a physical Wring Out to just making it Body Slam with 90 Base Power.
