Defeat 100 enemies with the knife in the main campaign
Resident Evil Revelations 2 – KNIFIN AROUND Achievement & Trophy Guide
In the room that needs the rusty key will be a lot of spiders which are easily killed with the knife. You can kill all of them and reload the checkpoint. You will respawn in the room with the key nearby. Run back into the spider room and repeat this until you have all 100 enemy kills.