Assuming completing the adventure involves simply opening Acerak's tomb and pillaging it, a 1st level party will suffice. But if completing the adventure involves actually destroying the demi-lich, then you need a 14th level party and in my opinion, you also need to cheat. Acerak's weaknesses are so obscure and arbitrary, his invunerabilities so numerous, and the rate in which he destroys characters without a saving throw so high, that unless the players had read the adventure before hand, they'd have to rely on stumbling on to right tactics literally by accident and they simply could not survive long enough to do this. The only way to beat Acerak is to beat him on the first or perhaps second round, and to do that requires absolute preparation and perfect tactics on the part of everyone in the party. You either deal the 50 hitpoints of damage (via obscure means) or the majority of it in the first round, are you are never going to.