Our group uses same or similar setup as yours and here's what we do:

1. At start of last phase both tanks (if both PLD) use Hallowed ground. WHM should hit Shroud of Saints. SCH focuses on healing MT, WHM is on OT and rest of the group (WHM should of course regen the MT and assist in healing them if needed and use Virus on a DN). When Hallowed wears, our main tank uses tank LB (1 bar), then rest of her/his CDs when that's done. MT gets both DNs, OT gets knight, soldier and at least 1 bug. One bug can be fed to the new DN (all other pt members stack next to where it pops so that the bug will run to them and DN will get it) or OT can take it.

2. Our melee stays on the DN we were attacking in last phase. Ranged DDs pull and kill the Rook asap. When rook is dead. Ranged DDs help MNK to kill the DN at low HP. Meanwhile OT kites soldier, knight and bug(s) further away.

3. When first DN is dead, magic DDs kill soldier and physical DDs kill the knight. After that they kill the remaining bug(s).

4. Kill the last DN and win.

Good luck!