252 HP/252 SpD Gentle Rayquaza survives slightly less than 50% of the time. If it has a choice scarf, it will always 2HKO with thunder (provided both hit).Okay, the closest I can now find is:
Kyogre @ Scarf
Modest, 128 Hp.|212 Sp. Att.|168 Def.
-Ice Beam
It OHKOs 100% of the time if Ray runs 256 defensive EVs and is an easy 2HKO with max EVs and a positive nature (it can OHKO, 37.7% of the time, but it does not need to.)
If Ray runs 256 defensive EVs and has a sash, none of +1 Outrage, Draco Meteor + Extremespeed, or Thunder can OHKO, and so obviously a Scarf set will not work.
If Ray runs max HP. and Sp. Def. with Sassy and holds a plate (LO will lose to recoil), Draco Meteor + Extremespeed and Thunder do not OHKO. If it holds a Choice Band, Outrage does not OHKO, as above, and Extremespeed obviously does not 2HKO, but if it holds Specs, neither Thunder nor Draco Meteor OHKO.
What have I missed this time?
Thunder followed by Outrage also does a minimum of 356, so it doesn't KO 100% of the time, but Thunder also only has to hit once instead of twice.