Zenless Zone Zero: Thunderclap Hound Emperor

Armor Hero One + Zenless Zone Zero.

Xi Shuo transmigrates into the body of an experimental subject belonging to the Praise Society in Zenless Zone Zero. The Praise Society intends to train him into a top-tier assassin, but he repeatedly fails during training and is punished with electric shocks for years on end.

Yet after years of torment, the Armor System awakens and activates the Golden Shadow bloodline. The Snow Mastiff Armor senses its summoner is in danger and automatically shields him, slaughtering everyone as he escapes from the Praise Society.

But Xi Shuo isn’t content just to escape. He sets out on the path of revenge against the Praise Society.

(A note: this is a coming-of-age story. The protagonist isn’t an overpowered wish-fulfillment hero. He may be pretty useless at first, but he’ll gradually grow stronger, and things will improve later on.)