FormerlyHemlock
Hero
Note: The Shield spell often does not last close to a full round. It only lasts a full round if the Dragon uses it on the PC's init immediately next after the Dragon's. Otherwise, it often lasts way less than a round. In our Dragon encounter yesterday, there were 7 PCs, 4 monsters, and a Dragon. 12 inits. The Dragon's init was 2 creatures below the Wizard's init, so its Shield spell lasts for 2 inits out of 12 or 1/6th of a round (2/7th of PC's inits).
A distinction without a difference in DPR analysis. Since the Shield spell can be activated on any hit, it doesn't matter if an attack missed against its AC 22 and its AC only later becomes AC 27. What matters is that you have to hit AC 27 or prevent/counterspell/dispel the Shield in order to hit it.