PsychoIncarnate
The Eternal Will of the Swarm
I just remembered I DO have a modest synchronizerI would suggest giving another Pokemon False Swipe for when Hold Back guy gets KOed. Just making sure, do you have a modest synchronizer in the front of your party? Having a Milotic might be useful...it would resist Ice Beam and Origin Pulse. As long as Kyogre is Modest, Body Slam shouldn't do too much. Activate Aqua Ring so that you can still slowly heal to last longer. I also doubt Kyogre would be OHKOed by a Milotic if it uses a special water attack.
That's a suggestion for if you have a Milotic or have access to one through trading with someone else or something. If not, Feebas is caught on that one route and has a 100% encounter rate if you fish under a certain bridge during the day. Just see if someone is willing to attach a Prism Scale and trade back to you.
and a relatively high leveled one
I used a gardevoir for most of the game until I replaced it recently