Think coverage and duration instead. Let's say a flask of holy water affects 1/16th of the lich, and it's holy energies are expended in that first round. Total immersion could thus potentially do a maximum of 16x the damage - and if the water kept changing each round, repeatedly.
Buy your barrel (I will even give you a STEEP discount for "no flasks"), but: It's not just quantity, it's deployment.
