I think with assassin it's better keep Dracul when you reach high level, basically because you can't use lifesteal efficiently, every enemy, lower than you, will be killed and so you can't have lifesteal farm. In the other hand, you will have many bloodpools and before convert to Dracul you can drink all your hp potions. Specially in this map where there are a quite number of undead.
In my run, i was faithless until the fight against the drake boss (after i killed the shadow boss) and convert to Dracul only to kill the boss (i was out of blackspace), taking 2 boons of blood tithe and using blood curse to kill the lvl8 armor in one shot. Actually, i could kill the boss only with my health potion (i had prep keg of health potion), but i preferred use Dracul, to be safe