I personally use the move once every 3 games or so, and i've seen other Bowsers use the move a lot more successfully then me, so these aren't all the setups you can get. Here are the rare situations I try to use it in:
* If I know they will roll behind me
* If you read a ledge getup->shield/spotdodge. The timing here is difficult but i'll try it sometimes
* If you read a ledge get up attack. This one is super easy but the problem is you need to be far to not get hit by the attack, and also close enough for them to think they can hit you. So like the only times I've hit this is when I dash towards the edge, stop before attack range, and then immediately forward smash
* You are on the edge and read a roll. This one is a piece of cake and I get this one all the time.
* You are dashing towards someone and read a spot dodge. This one is ballsy. I usually prefer to do an up smash because you have two hits in case you mistime the first one.
* tech chasing especially if you think they will attack out of a missed tech.
* if someone is perfectly spaced from you to hit them with the just the tip of the feet, i sometimes do a YOLO fsmash. Especially if you are a playing a heavy "footsie" type opponent and they happen to forget that spacing isn't exactly safe. People will just throw out pokes so it's kind of a frame trap.
* punishing incredibly slow moves. See
http://smashboards.com/threads/deal-with-it-a-guide-to-punishing-wip.381907/ for a parital list of moves you can fsmash
* punishing landings, especially if they mispace an unsafe aerial.
* punishing ledge jumps. This one is awesome, a lot of people are kind addicted to do like a fair from the ledge or something
* punishing opponents who don't ledge snap. I've punished so many shulks and luigis like this. Ive even shielded shulk's up b on the ledge and had enough time to fsmash them.
* i've seen people like mr. eh do jab1->fsmash, but i'm not crazy enough to do that.
* one option that i don't use as much as i should but i'm trying to make it my bread and butter is pivot fsmash. if you know they are going for a dash grab or something you can often pivot and start your fsmash before they've even committed to their grab.
so yeah. Even with this giant list, i barely use it. It's hard to get reads that I feel I want to commit THAT hard to.