Hmm, you probably already know what I'm about to type up, but hell, I got the urge to ramble on about something in case some lurkers out there are less experienced...
So manually soloing expert is fundamentally composed of:
1. How much healing you have (particularly party-wide) compared against how much you can lower normal overdrive damage. That determines how long you can last; theoretically forever as long as your healing's good enough. +healing is critical here, although that's dependent on the union cooperating. -atk is also crucial as always here.
2. How reliably you can force the demon to do normal overdrives instead of raging; preferably at least 5 turns in between overdrives, as that's the cooldown for the Andromeda line of souls' healing spells. Of course, swap normal and raging around if the raging overdrive is the preferred one to be hit by. But for Envy, you want normal.
Being able to reliably force a mode change (ie normal->raging and raging->stun) in one round of regular attacks is the crucial first step here. This one's basically an issue of your stats and how much -def you're packing.
After that, you'll need either overdrive meter reduction skills (but not a lot of kamihime have those) and/or direct damage skills (plenty of these on the other hand!). You want the direct damage stuff to be able to hit hard enough to force the demon out of stun. Why out of stun? Regular attacking a boss out of stun for them to then take a turn still advances their overdrive meter by one orb. Using abilities to wake up a boss from stun means that a turn hasn't passed yet, so that the overdrive meter doesn't fill yet. Having the option to do either should allow you to decide whether the next overdrive to happen will be normal or raging. Direct damage skills can still be used to help you force the other mode changes, of course. It's about planning head 2-3 turns in advance. Unfortunately, dazzle can really screw with predictions here :/