The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 8 (light novel) — Rui Tsukiyo

Lugh barely eked out a victory against his friend Naoise, who turned against humanity. Lugh swears to put a stop to the snake demon that orchestrated this tragedy between friends. He heads for the warfront, and crosses paths with someone he'd met in Dia's homeland. Setanta wants a rematch, but Lugh's planning an extremely long-range sniper shot.