It's a reaction ability that doesn't show up on unit lists and does not appear to have a limit. I'm over-levelled for the battles I am undertaking, and would be having no trouble at all. There are other, better, ways to add challenge to the game.
Any time you come across a talent that is unfamiliar, and proving difficult, my first recommendation is to try a different mix of companions than your usual tactics dictate. Rotate between ranged, melee, and magic companions.
Through much of the game you may find yourself having "no trouble at all" and this often leads to bad habits: getting stuck in a rut. Don't keep bashing your head against the wall trying the same old tactics you were skating along with.