Nanaly is considered the most broken unit in the game. At least, for sure in our version. There might be a unit or two in JP that comes really close or even passes her brokenness. If she isn't taking out armors for you, then it is simply because for one - archers are not anti-armors, and two - she needs to be 50CC50+ to truly be a powerhouse. When CC, she fires 4 arrows at once with her skill active, and when we get skill? awakening - that goes up to 5 arrows.

Archers are already very powerful units. But she is just in a league of her own. I've had her for a while on my second account, and she seriously melts everything including boss-type enemies.
Considered next best would be Spica, which you can get from the trading post. But her awesomeness doesn't start until awakening and skill awakening for her are available.