In my FC (our comp isn't mix/maxed, we are PLD+PLD+WHM+SCH+BRD+BRD+BLM+MNK), we're giving at least one GCD to the OT. The add does moves to a healer, doesn't touch him and goes back to the tank.

The 2 BRD put their 2 dots on Melusine and switch to the add.
The BLM cast one more fire to Melusine, sometimes add a proc then switch to the add.
The MNK finishes his combo and switch to the add.

If one healer is targetted by the shriek, we kill the add just before the 2nd spawns. If a DPS is targetted, sometimes the add has 5% HP left when the 2nd spawns.