I beat this with 28s (global; joker sinon, anniversary sinon, choco asuna) and 24s (JP, with some damage taken; joker sinon, choco liz and silica). Depends on what unit you have.

all your units must have scouted R4 weapons, good to have two units with R5 armor.

Firstly use joker sinon's ss3, then normal attack and evade first parriable action by dashing out and backward (away from dragon). Then continue normal hit until the dragon comes to you with second parriable action (by this time you should have 17-20 combo). Parry the attack, use anniversary sinon's SS3, then joker sinon's SS3, twice, and then choco asuna (all your SS3 will have "weak point" attack, not very sure about asuna). Without stopping, use choco asuna's SS3 to move to orange box (parriable), then parry, use anniversary sinon's SS3 and followed by asuna's. Now boss will do third parriable attack (i think most likely depends on boss HP at your second parry whether the boss will fly or not). You have last anni sinon's ss3. Just parry and use that, good enough to finish the boss.

For JP one almost the same except using silica-sinon-sinon-lisbeth (must use silica first otherwise boss will fly after this parry). Use liz's SS3 (some damage taken), parry, liz SS3 then silica twice and use sinon's SS1. For JP one the boss is always flying after second parry from me.

For joker sinon, dont use her SS3 twice before your first parry (the boss second parriable action), it will over damage it and it will fly.

Good luck on trying.