No, I'm not talking about the Traveller. I was pointing out the Pyro Vision Wielder from the Adventurer's Guild; Bennett.
Here are things to prioritize to maximize the power of Bennett to your team.
Bennett's ultimate ATK% Bonus will be based on his base attack. In order to increase his base attack, you need to do the following things;
Level-up - Every time you level your character, it will increase Bennett's base attack, same goes with his HP that will also increase his healing capability.
Artifacts - If you're prioritizing the DMG Buff of Bennett, increase the Plume of Death piece first before the others. Weapon - the easiest weapon for Bennett is the Prototype Rancour. Because the weapon's high base attack will significantly increase Bennett's DMG Bonus after he uses his Elemental Burst.
Additional Tips: Make sure you have at least Constellation 1 for Bennett to remove the restriction of the DMG Bonus. Any ATK% sub-stat will not affect Bennett's DMG Bonus, so make sure that you will aim for HP and Energy Recharge while farming for artifacts.