You have to set up an event and add conditions to make your bad guy throw an object at the time you want.
ths condition could be based on distance, line of sight, or whatever else you think is important.
(event and conditions)
badguy is mirrored
system compare two values: player.x < badguy.x
System compare two values: distance(player.x,player.y,badguy.x,badguy.y)< 200
system every 0.2 seconds
- badguy spawn bullet
- bullet set angle towards player