To scare someone really badly, trigger his fight-or-flight response. A severe form of stress, such as the thought of an eminent threat, can trigger the response. Being chased or threatened by a fear is enough to scare many people badly. Use a person's worst fears for the strongest reaction.
Feeling helpless also induces major stress. Create a strong scare by cornering someone in an area he is unable to flee from. Enhance the scare with a prop such as a chainsaw, fake spiders or similar fearful items.
A simple and classic way to scare someone is to hide in an unseen area and jump out suddenly. Use a closet, crouch behind a piece of furniture or quietly hide beneath a bed to perform this scare. Leap out to scare the target at an opportune time. Scare someone in the shower by sneaking up with a scary prop or simply shouting.
Technological devices are very effective tools when attempting to scare someone badly. Use a ghost drone, 3-D printed masks or a fake wound made out of latex to create the illusion of a person's worst fears come to life. Rubber masks and costumes frequently scare people, along with telling someone a spooky story or showing a horror movie.