In the event layer, right-click and select 'New Event'. Since you want to have the enemy actually be visible, you double-click in the blank space under 'Graphic' and select which graphic you want to use, that will allow the enemy to actually be visible. Next, in the blank space under 'Event Commands', right-click and select insert to bring up the 'Event Command List'. Select the third tab and on the top of the left column will be the Enemy Encounter button. That's so you can make an enemy appear. Then, to get the body to disappear after the battle, go back into the Event Command List. Select the first tab and in the left column, fourth from the bottom, will be the Erase Event button. That will get rid of the event, making the body disappear.