Hold down the DT button (L1 if default controls). The longer you hold the button,the more damage.However,the damage is equivalent to how many Devil Trigger gauge you have by buying Purple Orbs.The maximum you can buy is 6 or 5 orbs.It is quite expensive,but it is worth it.By using "Devil Trigger
Explosion" (I called the technique,not official name.),you can damage enemies that are surrounding you.The only thing is,that you Devil Trigger will be depleted to 3.For example,you bought 4 orbs,your devil trigger gauge will be 7.If you charge up to the maximum (hold L1 button for a long time),you will deplete 4 gauges in exchange for a DT explosion which will cause high damage,depends on how long you charge up.
Have fun.