For every lvl99 fight you get another spirit? That sounds insane. And what level do your spirits have that you can beat a lvl99 boss?
I'm from asia region, budda boss is the easiest among three
using average 4 points to kill lv99, I leave it for other players killing it if few thosands hp left.
Key strategy
1. Leader - Asura or other spirits with passive skill 2.5x first hit for all allies
2. Spirits with active skill stop enemy for 15s (but I'm only found 2 spirits are suitable, one is GR (SR) white body 50ap for 12s or asian water lady 65ap 15s, others needs 75ap), I'm using water lady, with full ap bar, I can stop boss for 45s total.
3. Full house 5 team members, prefer elemental day spirits. for a full bonus max uncommon spirit, first hit normally hit 1200
4. Recruit supporter, prefer Barong or high attack spirit
3 plus 4 is used for delay water lady spirit shown up too early
Good Luck