This is a questing deck. It takes a round or three to set up, but by round three or four it can post very big numbers.
The first task is to get Fireside Song on (MotK) Rosie Cotton, and that's usually what you mulligan for, but if you see enough other draw cards (e.g. An Unexpected Party or Drinking Song) if might be worth keeping. Of course, since you only start with one resource, you need to finagle that. If you also draw into a Song of Travel, play it on Rosie with Erestor's resource (if you can, get a Love of Tales on him first)) and then use the other two resources to put it on Rosie, and you'll quest for 9 on the first turn, sometimes more if you draw into one of your free songs (Song of Hope, Song of Healing). Another way to afford Fireside Song is to play a Legacy of Númenor, but that can drive the threat of the partner deck pretty high, so it depends on the quest.
The second task is to get Rosie able to boost herself. There are two ways to do this: Fast Hitch (which is fine, you'll want it eventually) and Let Us Sing Together, which with Goldberry let's Rosie quest without exhausting and boost herself. If you have a fireside song out, remember Let Us Sing Together boosts for 2 (one for the Fireside Song and one for its own effect). The Let Us Sing Together is particularly good, however, because it allows you to attach every event you play to Rosie for yet another point of . She can reach pretty stunning levels fairly quickly.
Finally, once you get two Fast Hitches on Rosie (which does take a few rounds) she can do one major defense or one major attack—but not both!—in a round. Sometimes I'll have her hold off an attack (either to deal with later or for the other deck to kill with ranged), but sometimes she can kill things. More than once I've drawn into a lucky Song of Slumber and canceled an attack and had Rosie take down a major boss, which is always fun. You can also do this if the other deck can provide a defense or feint, too.