The target monster only needs to be a Tuner when this effect is activated. The effect will still apply even if the monster stops being a Tuner before it resolves.
You can target a non-Tuner monster that’s being treated as a Tuner by Lightwave Tuning, etc.
If the targeted monster stops being face-up on the field, Barrier Resonator’s effect will no longer apply to it.
Barrier Resonator’s effect will still apply even if your opponent takes control of the monster it’s being applied to. In this case, your opponent would not take Battle Damage from battles involving the protected monster.