Mahou Shoujo o Muchakucha Taoshitai!

The protagonist, spending his everyday life at a leisurely pace, suddenly sees the death of a monster, defeated by a magical girl. "Crush the magical girl..." The protagonist inherits the desire of this dying monster and swears to knock the magical girl down. Thus, he gathers information to find out the true identity of the magical girl while watching her fight monsters. Surprisingly, her true identity turned out to be his classmate and head of the student council, Toudou Ichigo. She is absurdly strong, so to fight her would be a certain defeat. He devises plans such as attacking her before the transformation or making her fight lured monsters. While stalking, the protagonist finds out the weak points of the magical girl. First of all, her little sister, Toudou Lemon. There is also a mysterious childhood friend... And the fact that she is a magical girl is top secret. She is weak to dirty jokes and men who fall in love with her. Then, the protagonist finally discovers a fatal weak point. The more a magical girl uses magic, the more her sexual desire after cancelling a transformation will increase. The protagonist sets his heart on fire to knock down (violate) the undefeatable magical girl! [Translated from DLsite]