Per the description ingame:
If you use "Full Scale Attack," your Front Line will perform a single, powerful attack equal to the strength of the maximum number of attacks allowed by your ATK PWR plus a 20% damage bonus. Skills will only activate once. "Full Scale Attack" can only be used when your ATK PWR is full and is limited to 3 uses per day. The damage dealt will be carefully calculated and added into your total damage.
- If the Boss' health is lower than the damage dealt, the excess damage will not be calculated into your total damage.
- Full Scale Attack limit resets at 8:00 am (PST) daily.
This feature was first used in the One for All, All for One Event and has also been a part of the Never Never Land Event and the God Smack Event. Presumably, this feature will be available in all future Raid events.