Phantom is returning boss from DMC1, where you will fight him in Mission 14 as Dante and Lucia in Mission 10. Similar to Arius this boss can be random at times that can affect the speed of your quick kill cycle.
NG+ Note: Round Trip / Aerial Cross + Frost Heart trivializes this fight.
Your DT gague must be past 3 lines to get the optimal cycle. Both characters has similar idea. Stun Phantom once then use highest damaging move.
Like most bosses in this game there are some dangers to worry about :
This is how the fight should go with Dante. This is the best case scenario, where Phantom just stays idle.
Phantom with Lucia. Only consume the whole DT gauge with Aerial Crosses once the health is low enough to kill.