Tiamat and spell immunity, DnD 5e

The stat sheet for Tiamat says she can ignore spells of 6th level or lower. If I were to cast a lower-level spell, say Magic Missile, but use a 7th level spell slot to cast it, would it count as a 7th level spell, and damage her? Or would it count as a 1st level spell and fail to damage her?

It counts as the spell slot level used I believe. So it would be 7th level.

From the 5E SRD:

Quote:

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Casting a Spell at a Higher Level
When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level. Effectively, the spell expands to fill the slot it is put into.

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This can matter for counterspell and dispel magic as well.

But not for "Globe of Invulnerability", because who needs logic and consistency in a ruleset? *shrug*