Saijaku Shoku Ga Zense No Chishiki De Sekai Saikyou

A young wizard dominates with the knowledge from his past life, breaking through level limits! The strongest wizard, Roy, is reincarnated into the future as a young boy, Yuug. However, magic has declined since then, and wizards are considered the weakest class. With the knowledge of magic from his past life and his extreme-level abilities, Yuug embarks on his second life's adventure... (Synopsis taken from the raw pixiv site)