18+ Bokusatsu Tenshi Dokuro-chan

Bokusatsu Tenshi Dokuro-chan tells the story of a 13-year-old junior high-schooler, Sakura Kusakabe, who, twenty years in the future, while attempting to create a "Pedophile's World" forces all women to stop aging after the age of twelve, offends God by the side effect of having created immortality.

Dokuro Mitsukai, a member of an order of assassin angels that are called Rurutie, has been sent from the future to kill him. Believing that Sakura can be redeemed, Dokuro decides instead to keep Sakura so occupied that he can never develop the immortality technology. Due to her impulsive nature and super-human strength however, she frequently kills him with her gigantic spiked club, Excaliborg, on a moment's impulse, but returns him to life moments later with angelic power. Sabato, another assassin of the Rurutie order who uses her feminine wiles to do her job, is dispatched to complete Dokuro's original mission: Sakura's assassination.

(Source: Wikipedia)