It's about how you position your line up. The casting skill order normally will go from the front -> middle -> last along with move 1 2 3 4. for example: If you put your midnight blade with a sealing slash on X X Main highest row, your midnight blade will always cast your spell faster than your opponent if they don't have the same position. If they have the same position, it'll be depended on which character's initiative point is higher.

I figured this out when I tried to do Ninja Exam level 45, figuring how to stop neji's acupuncture and lee's inner gate.