-Flame Pillar is a useful mix-up in place of Forward Air or to hit airborne opponents at an angle you can't hit with F-Air
-Shorthop retreats
-B-Reverse momentum in the air
-Use it while returning to stage to deny the enemy access to the ledge to avoid being camped
-Mii Height determines how far the flame pillar will travel when used on the ground
-One thing to keep in mind is that the hitbox is not as high as the flame appears to be. The hitbox is a sort of white/yellow 'spike' inside the flame itself and it gets smaller as the move goes on, aka the hitbox is largest when the move comes out.
It can also be used for turning around instantly off of platforms by pivoting into the move:
http://smashboards.com/threads/flame-slide-flame-cancel-flame-pillar-at.378524/ This is most useful for ledge-trumping, as it allows instant-snapping to a ledge after running off the stage.
Ledge stuff~
When used on the ledge, the flame pillar can hit opponents before they grab the ledge for free damage while they try to recover. This can hit enemies through the stage itself - Be accurate with this, as the hitbox of the move is larger as it comes out so it is more likely to hit opponents at this time.
You can also combo it into a fully-charged charge shot at around 90-110% depending on enemy fall speed and weight for an early kill, though if they're higher percent than that they will fly above the charge shot (You might be able to catch them by short-hopping immediately while releasing the charge shot however) The flame pillar on ledge will arc them right into the charge shot and they can't dodge or jump.
Alternatively, you can train your opponent to recover past the flame pillar in a particular way to get onto stage, then punish for a kill when they're at kill percents. Most opponents I have come across will either roll from ledge to stage to recover, or they flat out avoid recovering to the ledge because the flame pillar ledge camp is too dangerous.