May I ask why, Wes? If he even leaves one type open, then that completely negates the advantage of being invisible to the dozens of others, making that ability useless.
On the other hand, someone can adapt their tactics to that particular aspect...say, by looking at effects of the invisible opponent on the environment (footprints on soft surfaces, a breeze as it passes by plants, etc.), which has been done in many other fights.
Personnel untrained in fighting stealthed opponents would have problems, but those who think on their feet would have a better chance. Or are you worried about things you can't see? Your characters won't have any problems, and I'm sure this won't be abused so no one else will raise objections. As it is, you're the only one to have any beef with it at all.