Define one-shot. If you mean that AQ5 last boss Overdrive one-shots you, then yes, that ALWAYS happens. You will need some serious shit to not die to it. It's a DPS race fight, though tanking is an alternative if you have the Hime to support that (though I guess technically Joan + Kaiser + -10% Eidolon would be enough, but uuuh...).
Thankfully, you don't need to beat AQ5. AQ4 also guarantees you an SSR each run, and is thus almost as good. Sure you can't find Ancients, but no big deal.
As for Ults, they vary a lot depending on element. Which ones are you having trouble with?
Anyway, as a general rule, Kamihime doesn't start for you until you have a full Grid of slvl20 SR Assault weapons (or better) and full level 100 SSR Eidolons in your Eidolon Grid. That's the starting line. After that, it's a matter of events, events and events to get stronger. Kamihime is a marathon game, you can't really rush it.NEVER do this. The average SSR Accessory is complete trash, so you're literally just throwing your Medals away.Yukimura does nothing. The only purpose of Yukimura is getting Shingen, so until you have Medals to reach her, NEVER do this.
After you can do it that way... ... uuuh... still probably not a good idea? But up to you, I guess.Usually not a good deal at all.
Honestly, the only good deal from Medals are the bricks. Try to save up for those, as they're your final roadblock in Kamihime progression and not really obtainable through other means.