The main reason I added the immunities was I needed to be able to ignore stagger since this only triggered at low/negative health. If others agree with you, I'll change it though, and I want a spell effect other than haste. With ravager's rush limited in its uses, it won't give the biskmatar too many extra turns. The spellcraft check for siphoning touch was necessary, I agree, especially since it allows casting out to close range. (though the biskmatar shouldn't be more than 25 feet away from the caster anyway.)
And although he's best when supported by a wizard, he's still pretty good in a tactics/leadership role if he's buffing himself with wands/scrolls.