With Innocent the enemy Summoner will not be affected by Zodiac, which prevents him from learning the spell. Try lowering Faith a bit (yours or the enemy's), casting Shell on the enemy, or Mind Breaking yourself, so you don't kill the Summoner.

Once the Summoner has learned Zodiac, either wait for him to cast it back at you, or kill him and absorb his crystal.

Check out Master Daravon's Zodiac Learning FAQ over at gamefaqs for a more detailed explanation.