Growth Ray for Sam
Temporal Watch for Red
Shrink Ray for Vix
Have at least 16m xp on all characters and minimum 10 to 20k hp on all.
On the first stage of the fight with the Fallen Queen Slime, kill the Queen and the healer, and leave the mage alone with low hp. Once the mage only steals xp but doesn't interact with the Queen nor the healer, start farming hp by mending yourself over and over again.
The two other Fallen bosses shouldn't be hard and when you reach the fifth and final stage, grow Red, shrink Miss Darkness and focus all the damage to her until she's dead. Use Sam only for mending and healing, never for attacking, and Vix for shrinking the kitsunes and Miss Darkness, never for anything else. Red will outdamae the healing and kill miss darkness with a full team attack (if it fails, you gotta set up again).
Alternatively, Dragon's Throat bypasses the Immune effect, so if you have an enemy that constantly heals, you can use that item to deal 200dps max and slowly deal 99999 points of damage.