after you narrow down your pick to all tanks suitable for attack add a subevent with
pick nth instance (0)
and do the actions on it
if its still switching, maybe you need to lock your target until it dies or for a certain amount of time
add another var on your turret attackUID=0
now while it's 0 you're picking a tank, once you picked you're updating this value to the tank uid
than you need to reset it to 0 once the tank is destroyed or in any other condition you like.