Insightful Vision

Seeing two Mana-infused slugs strike the ground rather than into his target had filled Henry with an intense feeling of loss and thus, to mitigate any more such losses —which were likely considering his opponent's speed—, he needed a cheaper option. 

It had cost Henry 60 Mana points to use <Guiding Shot> on every single one of the bullets he fired from his normal revolver but he was satisfied with the cost. 

Missing shot after shot with 'The Hydra's Sting' would have cost even more Mana Points than that and missing with 'Violent Retort' would have cost him Mana-infused Slugs of which his supply was currently limited. 

'The Hydra's Sting' being a magical weapon was capable of shooting faster than the normal average revolver but even that faster firing speed did not fill Henry with confidence. Which was the reason for this experiment of sorts with bullets he knew could do nothing against the Undead Knight.