can you be more specific? Like do you mean run up to the monster and attack? If so, you need to go to the item tab, pick the weapon that person is using then you'll see a black box with the word "Animation" over it. Then hit the "Set" button. On this menu at the top left-ish area you can pick wether the player: Steps foward to attack, Jumps foward, no movement, or runs up to the monster to attack.
If that isn't what you ment then I don't know what you mean by "attack a sertain monster when the player steps on a position".