I agree with Stickmasterluke: momentum should break it. However, "Shield: All damage is blocked. Any creature or weapon with 8 or more attack or momentum destroys this shield when attacking." is way too complicated. You could rephrase it:
"Shield: reduce damage by 7. Breaks whenever you are dealt physical damage"
Since damage is reduced by 7, only a creature (or weapon) with 8 or more attack (or momentum) can break the shield. The only difference being that a creature with 7+n attack will deal n damage instead of 0 when breaking the shield.
I also agree with Rutarete: 8|9 damage to break the shield is way too much, you should lower it to 6|7 (or, with the wording I suggested, damage reduction of 5|6).