Equip the Coin Of Plutus relic, which increases the time available between hits in a combo. Complete the challenge where you must stay airborne for eight seconds in the Fraud circle of Hell. This challenge spawns low level enemies. Blast them with the Crucifix attack until you get a 666-hit combo. Note: Do not use finishing moves.
Another way to obtain this trophy is by completing the game and, once you get to the 11 trials, make sure your Cross (from killing Dante's Father) is fully upgraded. Sswitch your difficulty to Infernal and on the Infinite Mana challenge, simply use the cross repeatedly on the monsters until the challenge is over. If correctly done, you will have about 700 or near that hit combo.