Gold Keeper turns to red at half HP though so it wouldn't have mattered anyway. It does stop Unit-13 teams though.
Edit: Or not? You can use 4 35% gravities, do a couple attacks, take a hit and then finish it off. But the pre-emptive attacks would make the team hard to work anyway
You wouldn't use MS to kill any of the pre-boss Keepers. They all have a minimum of 3 turns between each attack, which is enough time to fully heal between even their special attacks, provided you have enough RCV and HP to survive the attacks. You do need a high level team plus +RCV, though, since you'd need to break 65K HP to survive a RK attack and all the 35% Dark gravity monsters(only Stratios and Zaerog currently) have negative RCV. As such, it's purely a theoretical team that would not be worth the effort to build, considering the low chance of even killing the GK before it does its mega attack.
Grodin + ultimate Orochi on a ~20K HP team can survive GK's special attack, but the problem is fully healing between attacks. Same with Michael/Michael and 3 max-skill ultimate Susanos.
I'm not sure if anyone will come up with an auto-win defensive team. At first glance, it seems like some type of spike team will be the best solution. Maybe a DQ Hera team with max-skilled gravities and King Baddie. Use up all turns on each Keeper to have gravities charged in time for GK.
A quick team idea:
DQ Hera
Any high skill gravity mob
Persephone/Ronia/etc.
DA Lucifer (Fully awoken he's amazing, and skilled-up he's redonkulous. Can even be used twice on GK at max-skill)
King Baddie
DQ Hera
3 gravities bring GK down to 2,406,645 HP, which King Baddie should easily 1-shot. Even just 2 gravities to bring it down to 3,438,064 is killable. The sub monster slots are very flexible.
edit: Looks like people aren't sure whether GK has a split F/L life bar or not. Should still be doable even if it goes Fire at less than 50%, but obviously not as sure of a thing.