Debuff Master is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Han Taesung, a livelihood-type gamer cornered at the edge of a cliff. “I have been watching you for a long time.” Deus, a hidden NPC of level 999, appears In front of Taesung! [Would you like to become the disciple of the hidden NPC Deus?] [Yes!] Thus the master’s teachings began!! “Two blows are not cool, you must defeat enemies in one blow.” Taesung becomes a who knocks down enemies in single blow. Everyone will fall prey! Defeat them in one hit!