Please note: all credit goes to Fox!
There’s a spot on Ouranos Island right next to Big’s fishing portal, with three floating platforms that can be hit. After a bit, you’ll be able to activate Phantom Rush off them. Rinse and repeat! No need to start a new game.
Guide to Obtain The Combo Crackerjack Achievement Post-Game
Ouranos Island Floating Platforms
Go to this area on the Ouranos Island map and find three floating platforms. It’s slow, but you can activate Phantom Rush off them by using a homing attack.