Beating a Demi-Lich[edit]
To beat a Demi-Lich, you need to either get a mage or a Barbarian with +5 Weapons, then use the Barbarian's rage or have the mage cast immunity to abjuration then go in and slaughter it because that will make you immune to his Imprison spell.
The Staff of the Magi[edit]
The Staff of the Magi is probably the most powerful staff in the game. You can get it by having a Rogue Stone and going to the door right of the temple of Waukeen, there will be a door to the left of it which is trapped. Don't take that one, take the one to the south of it, you will be teleported to a weird plane. Prepare for a battle with two powerful spellcasters, a Beholder, and a few more enemies--Layene is carrying The Staff of The Magi. To get out, put the Beholder's Eyestalk in the pot to the right. The staff counts as a +5 weapon and provides Protection from Evil, Invisibility, +2 AC, and allows you to cast Spelltrap 1x/day.
A solo sorcerer can win this battle at level 14, provided he chose Finger of Death. Cone of Cold or Sunfire may be necessary in order to bypass Layene's Globe of Invulnerability and interrupt her Time Stop.