Very hard mission that took me several tries. Initially I had to sacrifice a healer to get one of my strong defense/hp characters to the triangle that buffs the enemies 3X. Getting to that and destroying it was the first objective.
I left some other strong team members undeployed so I could bring them out after the buffs were removed.
However afterward I had issues with 2 enemies that would not take damage from regular and special attacks. I had to end up using giga fire again and again which was very tedious and I almost ran out of magic replenishing items.
There is probably another way to fight them. Maybe Ice magic but I didnt have it.