Cadence is basically the magical incarnation of love. Love and friendship are kind of a HUGE part of the My Little Pony world, and if Alicorns represent a very key part of life (Day, Night, Love), then it's no real surprise that the sheer power of Cadences's love gun could almost/even rival Celestia's... Day... Powers...
Plus, Twilight Sparkle can cast bubbles around her friends, and a tree library... Shining Armour can cast a shield around an entire freaking city! I mean, with the combined powers of the love of an alicorn, and sheer defensive force of the magical Captain of the Guard, it's not /too/ much of a stretch to imagine they could do away with Queen Chrysalis.
Also, thinking of it, Chrysalis said she was being powered by the love of Shining Armour... and with that amount of love (and whatever pre-existing strength a Queen Changling would have) she defeated Celestia. Her strength didn't draw from Cadence or seemingly anything else, it just seems like Shining Armour REALLY loved Cadence aha. If Cadence were to have taken Chrysalis on horn-to-horn, they would've tied, but with couple together... yeah... I think I'm caring too much about this particular point. :I