Zi'Assunta is unlocked either finding her coffin in Cappella Magna and then buying her for 5,000 gold or by using the Forbidden Scrolls of Morbane and casting the spell "paradigmshift".
From my understanding using the secrets menu doesn't disable any achievements but I believe you might still need to go grab the coffin regardless for the following achievement:
By this point of the game the 15 mins should be trivial for you, Zi has a .5% buff every level for Might, Projectile Speed, Duration and Area but imo the whip she starts with kinda sucks. Regardless, use whatever combo of weapons you prefer and remember to use "Hurry" before starting your stage to cut the time requirement down to 7.5 mins and then quit once the timer hits 15.
