Savanna game - the comic manga

Savanna Game: The Comic summary:

The story follows a young man named Kazuya Shibuya who receives an anonymous email message out of the blue one day: "Won't you join the Savanna Game?" The message is an invitation to a state-sanctioned killing game — a deadly role-playing game designed to motivate the inhibited youth of modern Japan. Shibuya and his two friends Kotegawa and Kudou find themselves in bizarre battles that span the space-time continuum, with everything from dragons to the Shinsengumi force of the shogunate era.