When its done shooting the first target have it do a GetAllOverlappingActors, if you want it to prioritize at that point its kinda going to be up to you. You could do distance from tower to target, or if you have some kind of variable on the targets for how close they are to the end goal you could have it pick its target based on that. If you get all overlapping actors and there are no actors to attack then just have it sit there until one triggers the OnBeginOverlap again.