I find the boss fights so far to be pretty flexible in terms of the strategy you can incorporate really. It's just a matter of being fast enough on the draw with hot-swapping between roles, and in a lot of cases, if you don't appropriately time your change, you are going to die (barring of course, being over-"leveled").

I do really like that the boss fights haven't been a walk in the park, which mostly applies to (SPOILER)the Eidolons.

As for the boss of chapter 9, (SPOILER)I found out after my first death that just switching to relentless assault and pummeling him while he was charging destructo would result in him getting interrupted, it's just a matter of hitting him hard and fast enough. I used Lightning/Fang/Vanille and used Ravager-Syn-Syn and Ravager-Commando-Medic and of course Ravager/commando/ravager for pummeling.

The first phase of the fight is purely about living through the constant attacks (rav/com/med I found was best for me) then using rav/syn/syn at phase 2 onset, and sitting in rav/com/med until destructo started and going all out with rav/com/rav.

(SPOILER)Cid just has three distinct phases that you have to be on the ball to take on, (for me, I just used the same team) debuff and Lightning medic during guard phase, rav/com/med during attack and rav/com/rav during heal. I tend to use the summons as an emergency "Oh sh*t moment safety net," (like...when everyone is taking massive damage and the healer is silenced or stopped completely).

Pummeling with what hte mob is weak against after it's debuffed means a very, very quick stagger blitz, it's just a risk you take that does very well lead to random deaths from time to time when you choose to go all out with attacking...but geez that damage is beautiful. Ravager Lightning and Commando Fang are things of beauty.