Literally stated that Julius is one of the only people who can defeat a Devil.

Is Julius still powerful?

Even though his age is 13, his connection with the Grimoire from when first received it still exists. Although that reincarnation spell reduced his Grimoire to a single page, he is still capable of regaining that power.

How strong is Julius?

Abilities. Immense Speed: Julius is extremely fast, as shown when he instantaneously appears between two Eye of the Midnight Sun mages killing them before they can even react, and dodges attacks from Sally and George with ease. He is also able to outmaneuver Patolli, a Light Magic user, and dodge the elf's attacks.