I Kept Pressing the 100-Million-Year Button and Came Out on Top, Vol. 3 (manga) — Syuichi Tsukishima

After inadvertently wreaking havoc at the Elite Five Holy Festival, Allen has been suspended along with Lia and Rose, and the three are put to work as witch blades. Of course, a few goblins and chimeras are no match for Allen and his billion years' worth of sword training…but monsters aren't the only foes he'll have to cut down. An unexpected reunion awaits him on his mission at the Unity Festival -- and this ghost from Allen's past holds a grudge for him deeper than any other…