How is she killed? Right before Myrcella departs on the ship with Jaime, Ellaria Sand kisses her on the lips. At first, it just seems awkward, but once she has set sail, Myrcella gets a nosebleed and falls down. Poison!
Is she definitely dead? Yes. Though we don't actually see her die, the producers refer to poor Jaime as a "grieving father" on the "After the Episode" featurette.
Level of devastation: Seven. We never get to know Myrcella very well, but this death is particularly poignant because it occurs right after Myrcella tells Jaime that she knows he's her father, and that she's glad. It's his one moment of joy before she's taken from him. Sob.