The rules very clearly state that it cannot be moved, and seeing how it doesn't have a profile of any kind, it also can't be destroyed.

As for tactics - not moving obviously lends itself to a defensive strategy, which should combine well with Elven archery and Galadriel's Blinding Light. Note that this may not necessarily work in some scenarios (although e.g. putting the Mirror on the hill of The High Ground and posting some tough heroes nearby sounds like fun). To make good use of it, I'd say it's best combined with at least 2 heroes with at least 2 Fate. Mounted heroes (able to run to the mirror, and come back refreshed next turn) may work, but are much more likely to lose their mount before losing their Fate points. Galadriel herself, as well as e.g. Gandalf will be near unkillable within range: 3 re-rollable Fate that can replenished each turn. I can see it working nicely with an all-hero White Council army.

I've never faced an all hero army with the mirror but I think it actually negates some of the mirror's strengths. You can only replenish 1 hero per turn of their fate. And south so few units you might have multiple heroes needing it at once. Also even with full fate you can still die if you're surrounded and enough wounds are inflicted, having the regular warriors prevents your heroes from being that outnumbered in combats and IMO as a result boosts the mirror's effectiveness.