At that stage of the game, my usual tactic is to use:
Ami to stack Big Guard over and over until your max hp cannot increase further. Usually the team HP max can be pushed to 9,000+.
Main healer heals every turns to top the team's hp up.
The other two are the damage dealers.
My tips are:
1. Make sure Ami and the healer are faster than the boss. Ami is faster than the healer.
2. Big Guard HP buff last 3 turns, so when you HP max is enough, Ami can do other things for 2 turns before using Big guard again.