Wo! Shishang Zui Qiang Jin Zhou Shi

Ye Lin is transported to a world overrun by demons. On the day of his class advancement, he becomes an SSS-rank Forbidden Spell Mage—only to be met with laughter and ridicule. Why? Because casting forbidden spells consumes the user's own life force. It's the most powerful class, but also a guaranteed death sentence. What they don't know is Ye Lin's secret: he's immortal. The class's fatal flaw means absolutely nothing to him. When they witness Ye Lin casually spamming hundreds of forbidden spells at once, everyone completely loses their minds.

"Just how many lives do you HAVE?!"

"Are you seriously using ultimate-level spells as your basic attack?!"

(Source: Kuaikan, translated)