So I notice sometimes when a boss is on the verging of entering it's next phase. A crossed circle appears over its HP. I've looked up this phenomenon and although it is apparently a known thing, there is no answer as to what it means. As I'm attacking, my hits are still registering damage but because the boss's HP is so high, I can't tell if I'm still actually affecting it. Do these transitions count as momentary invincibility? Do I have to allow the transition to complete before I attacl again?

Also, I'm currently fighting Arsenal and I can't recall from previous playthroughs (and again this amazingly had no answer online) but do summons not occur during this battle? I've been fighting this thing for over an hour and the summon meter has not appeared. The last time I used a summon was against Jenova. What's going on?