right arrow to go forward, left arrow to go backwards, keep your mouse on the enemy. The red plus is the enemy's mouse. The lines are the weapons/ reach lines, and you will only attack if your mouse and line are on the enemy.
update 3.0, added faster speed for more intense combat, and added background and health indicator fading effect. Btw this is only meant for 2 people in the world to play at once, so it might be buggy if there is a 3rd person.