Berserker Mode is more likely the result of Miseng's hardworking training and determination to control the Shinsu. She is now able to bring out her real potential hidden power in Body Reinforcement field without having to loose her consciousness anymore.
When many Regulars has gathered to fight in the grand stadium, Miseng entered into unconscious state where she suddenly become very strong. She was seen to fight at least 4 male Regulars and defeated them easily. This happened after Verdi gave her a mysterious red flower.
Verdi requested Prince and Miseng to become her "doll". Verdi has a unique ability of Doll Making that will enhances the hidden power of a person. It might sounds like Verdi mind controlled her target like a doll, but it's seems very unlikely, because although Miseng was unconscious, she can gets provoked by Prince. Plus, Verdi wasn't seen controlling Miseng either.
Miseng was seen to utilize a Body Reinforcement technique where she can temporarily increased her speed and strength. She called it the Berserker Mode.
In the flashback story, Verdi complimented Miseng that she is extremely good at using intense burst of power.
- Shinsu Boost: Berserker Mode (LINE Webtoon)