‘Metz had no sooner shot with his pistol than with a nonchalant flick thrown it over his right shoulder, causing it to arc gracefully in the air.’

‘Shin evidently had a lot of confidence as he abandoned his weapon to the water, allowing Metz to straighten up as he jumped.’

'His left arm no longer low he could turn his right shoulder in the air to prepare the knife lunge.'

Also, Metz gun is -definitely- in the water, but whatever. 

'If only Shin knew how hilariously lucky he was that he started moving again.'

Quite. Although with absolutely no time scale to work with Metz might have just completed his spell long before your character remembered the firearm, it's fortunate he happened to move in the exact moment Metz started casting, despite remaining stationary for what I can only assume was up to half a minute.