Well, does each ghost have a "goal" it's trying to achieve? If it's just a floating target sign, then your players are just going to take turns shooting it until it's trapped. But if it has something it NEEDS, then it'll try to do that regardless of what the Ghostbusters are trying to do. If you need some inspiration, get a D6 and roll once for each category on this chart we made:
Is the ghost funny? Scary? Sad? Each situation can be different. Maybe the best way to stop the haunting is NOT to shoot it. Let them role play a discussion with it. Maybe through a seance?
If you want to it be more dangerous, that can be tough. Your ghost will only get one turn to do anything, but each member of your team will get to take a shot. To mix it up, have the environment react even if the ghost can't. If a player misses their shot and rolls a ghost, have a lighting fixture fall out of the ceiling, and now you team has to duck as it comes swinging by! Or if you hit it, the ghost goes through the wall, and into the ladies lockeroom next door. It's easy to just get focused on what the ghost and team are doing, but remember the environment can be just as much of a character.
The TYPES of powers a ghost has isn't as important as what it uses them for. Ghost can possess people to avoid being shot. What if instead, a ghost possessed a stray dog and tried to make a run for it? Now you've got a chase scene where several Ghostbusters are running down the street where anything can happen. Maybe some nearby people think you're trying to hurt a regular dog and try to stop you! Maybe the dog isn't a stray, and it's owner shows up not knowing their dog is possessed!
The point I'm trying to get to is that it's easy to accidentally fall into a routine of "You show up, you shoot it until it's weak, then you trap it. Hooray." So try to think of things in advance that could change or go wrong. Take the fight outside (or inside!) Have people stumble into the problem without knowing what's going on. Have the ghost randomly pick one Ghostbuster and only attack THEM. Or maybe the ghost has a secret ability it will only reveal if it thinks it's in real trouble. Like in The Princess Bride: "I am not left-handed!"
You'll never be able to predict what your players will want to do, but just plan out some possibilities ahead of time, and when the time is right, spring it on them!
Anyway, I've been rambling again. But if you need more inspiration, you can check out
the rest of our Investigation Chart here.
I hope this helped. Let us know how your next game goes!