Alastor can be dangerous for melles duo of some aspects ( i have his dbr file open right now)
-Alastor have HP and mana leech skills
-Alastor have Ghost/undead resistance files, wich mean, (50% Life / 1000% Life Leech / 1000% Bleeding / 1000% Poison res & 60% Physical+Pierce Absorb), Plus he have 10% physical and pierce base resistance, if you dont have good gear it can be messy.
-Also you face him , kinda early , lv 18 or so.