How is someone passing their magic to someone else a plot device if that's the case then you're saying natsu gajeel and future rogues dual element modes are plot devicesPlotsu and Plotray used PLOT! Mard Geer was too OP for them *****s. Gray new powers are a plot device.
Devil Slaying magic is only ONE way Fairy Tail could've beat Mard Geer. The writer could've decided NOT to make Laxus sick the whole arc so he could've helped against Mard, or even better, Gildarts could've come back. Devil Slaying magic wasn't a desperate plot attempt to make an invincible enemy lose, it was just a way of preparing Gray for his future fight with END.Plotsu and Plotray used PLOT! Mard Geer was too OP for them *****s. Gray new powers are a plot device.