For the union buffs I was told it caps out 400 (100 can go into each with 8 different choices) and resets everyday, so you can't do what you're saying with leftovers since you're already at the cap for the day with your picks.

Personal opinion on priorities:
1. Atk. Pretty much a necessity if you want to get anywhere.
2. HP. Extend survivability for more attacks.
3. Burst damage. Everyone can use burst and it's godsend during burst hours and Siegfried.
4. Triple Attack. More attacks and quicker burst gauge fill for more damage. RNG though.
5. Double Attack. Same as above, unless double attack has some higher success rate. JP wiki lists nothing.
6. Regen? Or is it heal increase? This can be quite useful as well but honestly a low priority since this isn't a solo job. Might good for inactive unions though.
7/8. Affliction rate buffs are fairly useless this fight.

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Could someone verify if the boss levels reset each day? Or is that just buff? I actually can't find where it's mentioned, only mentions of union buff reseting. I mean it seems a little impossible if it doesn't.