Jigoku Sensei Nube Neo: Chapter 102 to 104 [GTS/FMG]
 

Hi, the process starts from chapter 102, where the scientist girl is realeased from possession, then the demon merges with a monster summoned and thus becomes a GTS monster.
Later on(in chapter 104) when Baki and the remaining demons give her a hard time, she gains more power and an FMG powerup happens.
Ch102: https://lhscan.net/read-jigoku-sense...apter-102.html
Ch104: https://lhscan.net/read-jigoku-sense...apter-104.html