Tiamat is immune to bludgeoning from non-magical attacks.
Tiamat is immune to Acid, Cold, Fire, Lightning, Poison; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks. So she is totally immune to this spell. So is the Aspect of Tiamat and Bahumut.
Which when you add Terrasque means EVERY SINGLE CR30 monster published by WOTC is immune to this spell!