Levanna will use her reaction to cast Shield if an attack hits her.
I've played under DMs in real life that have done this in two ways. In the first, I get to know what the attack roll result was before I cast Shield.The other wouldn't announce the result until after I had decided whether or not to cast Shield. Obviously, in the first, I wouldn't cast the spell if the attack was going to be over the modified AC and in the second there were times when I cast the spell and it didn't matter; the attack was enough to hit anyway and the slot was "wasted". How would you like to handle it here?