！
And to top it off, his very first battle was that dragon slaying!
Literal
Moreover first-battle [topic-は(dropped)] that dragon slaying!
しかも 'moreover / on top of that' is the additive discourse marker for piling on something dramatic. 初陣 '(one's) first battle / debut' is a historical/dramatic compound, used in military and by extension sports/competitive contexts for a first outing. 退治 '(to) defeat / subjugate / exterminate' is the sinitic noun for monster-slaying in fantasy narrative (reinforced by RPG vocabulary). A sentence fragment — no finite verb — delivering an excited exclamation.