Yūsei Matsui’s Assassination classroom (published by Viz here in the US) has an outrageous premise even for a manga: an immensely powerful alien who has a smiley face head and an octopus body has come to destroy earth, but will spare the planet if he’s assassinated. He takes over a class of high schoolers and teaches them the ways of assassination…but he still proves difficult to kill. Even worse, he’s an awesome teacher and everyone’s grades get better!
Mausui’s art is a clean, surgical edge to it, that makes the violent goings on all the more effective.
Viz has a nice 52 page excerpt of Vol. 1 of this popular series available on their website. Certainly enough to see if it’s your cuppa.