Dancing Blade (Side B) is singlehandedly one of the best tools in the game for punishing rolls, spot dodges, landing air dodges, laggy landings, or just punishes in general. It does a lot of damage, ranging from anywhere around 16%-25% ish. It's main boon is the fact that you can do it out of a run. Marth doesn't have to commit to a Grab, Dash Attack, Aerial or Up Smash for his punish. The fact he can do Dancing Blade immediately out of a run is one of it's best traits.
Ftilt works if somebody rolls behind you, as it has the range to hit them. If somebody rolls into you, Utilt is a good option, as it can set them up for a combo or a juggle. If somebody rolls infront of you, try Dtilt, which you can follow up with a running Dancing Blade, Fair or even a grab at certain percent's.
A lot of players make the mistake with Marth of "Oh, I have an opportunity to punish the opponent, let me run in and dash attack/smash/Fair!". This is a bad option, for a few reasons. Not only do these options have next to no follow ups, you can get more damage and follow ups from Dancing Blade if you angle at the very least, the last hit upwards. Another thing, is you need Marth's main moves to be as clean/fresh as they can be for killing. As such, try not to use Fsmash for a close range punish, as tempting as it might be. If you accidentally land a sourspot at high percent, you need the best chance at it KO'ing.