The highest possible combo rating is an S+ which should come naturally during play. If not, it can be acquired easily. In a room with multiple enemies such as armoured Wights, activate Colchis' Trap III followed closely by a Mag Mell's Rose III cast without a target. This will cause Mag Mell's Rose to damage every enemy in the vicinity multiple times. If this didn't earn a S+ rating just follow up the Mag Mell's Rose with a Colchis' Trap II and use scythe attacks on the trapped enemy until you either get the S+ or get a finisher.