Easy "Demon Slayer - Gold" achievement
At the start of world 3, level 2 are two demon spawn sites that are separated with a set of doors. Collect the key from the pedestal and unlock those doors. Remain at those two spawn sites and kill as many demons as possible. You can also destroy those two spawn sites and continue by lowering the bridge to find another set of two spawn sites. Remain on the bridge and kill the demons.