it is best to have at least 3 people there. There are some obstacles that will be over with if there are more than two players in the dungeon. Besides, Malistaire is high rank and has like 10 thousand health points, it will take a long time for one player to defeat him... Good luck!
Actually it can be done with 2 people (if you know what your doing of course) Killed him several times with just me and a friend me fire and him ice schools