You have to cast it on an object yes... There is not save, the point of having light source (light, or dancing lights) is that is makes hiding harder
Not quite following you. Are you saying you cast Light on an object near the Matriarch (or other obscured enemy) rather than the enemy themself? I was picturing her all lit up so you could see her wherever she phases to, but of course that wouldn't work now I think about it as she won't be holding weapons, and I think you can only target armed creatures, in an attempt to apply Light to their equipped weapon.
I agree, if you're applying Light to an object in the environment there's no save, only when trying to light up an enemy's weapon. I admit, I've never really tried to do the latter, but I guess it might be worth a go on an enemy that was likely to try to hide, maybe?
