Tartaruga's Mighty Shell

touch one willing creature, this spell causes a corporeal turtle shell to appear on the targets back, granting that target the natural AC of a Tortle for the duration of the spell, changing the targets AC to a flat 17 (no dex calculated). The caster may not cast this spell on themselves and this spell can only be cast on one target at a time. If the target's AC already exceeds the AC of this spell when it is cast, the spell fails

 

At higher levels this spell lasts 2 minutes longer and grants +1 more AC at 5th, 9th, 12th, 16th, and 19th level