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Like so many before me have said, it's fair. Narrating it not only gives the players the "heads up" that a certain tactic might be in play, but it also raises the excitement of the encounter by adding some dynamics in.
During game preparation time, you could jot a few notes down to help guide your combat tactics.
For example: once they are below half health, they start defensively fighting. OR, at the start of combat, power attack is on until...
You don't need to stick to what's been written, but it gives you a reminder during the combat to consider this tactic.