I appreciated the difficulty in this one. It took me a lot of tries to win. Overcoming a hard boss that requires a bit of luck felt good. And the ending played out in a really satisfying way for me, with everyone except Diao going down and Diao barely surviving with something like 2 MP left.
I tried a lot of different strategies. My interpretation of the name "Sacrifice" was that you're supposed to sacrifice Rosia once she runs out of MP for Holy Magic.... It took me a while to realize that her physical attack is useful too. I picked up on the fact that Diao needs to be alive to cast Dark Magic on the angel phase pretty quickly. I ended up having Ultos and Diao defend during the first phase while Rosia and Stanow focused on attacking. I noticed Stanow had the highest speed, which was useful when I needed a heal to come out before the demon attacks.
I found this strategy of having 2 attackers and 2 defenders somewhat striking, and a little unintuitive. My initial thought was to damage race the boss with 3 attackers and just have Diao defend. But when Ultos joined in the attacking, I seemed to run out of resources too soon. It's possible I just got unlucky in those attempts, but maybe something about the damage numbers works out so that having 2 defenders means you spend less Potions on average to keep everyone topped up. I'm not sure I've fought an RPG boss before where I had to make that kind of calculation.