Boom got it! After multiple failed attempts @ 10% running pure turtle, got some advice from reddit and made a swap for Vaan and triple healer and worked out. I've uploaded the profile to
https://www.ffrkcentral.com/profile/9Fxy (ignore the RW, I used Deployment Tactic)
Basically:
1. Onion Knight - Full Break / Power Breakdown, BSB, Mako Might
2. Vaan - Life Siphon / Magic Breakdown, BSB, Ace Striker
3. Relm - Curada / Shellga, BSB, Lionheart
4. Vanille - Curaja / Protectga, BSB, Knight's Charge
5. Y'shtola - Wrath / Memento of Healing, Wall BSB, Dr. Mog's teachings
Starting is tricky because I couldn't spare a Mako Might on a healer, so I had to reset for S/L a few times for RNG to land Wall, Haste, Magic Breakdown, Pro/ Shell and then defend as many people as possible, slowly healing up until the BSB charges up. Saved deployment tactic till ~75% - 80% while applying breaks / Vaan dps. You don't take that much damage if you apply enough breaks for the counters to matter.
3rd Phase is frustrating, but with the SB gauge you stored just spam cures, guts and have vaan + OK just do DPS, after a few tries managed to pull through. Really needed to focus equipment on Def / Res due to all the armor ignoring attacks, so I augmented quite a few armor that helped a lot. Pretty sure it's doable with 2 healers but I needed Y'shtola for wall anyway so she doubled as a 3rd healer. Good luck guys!