On the other hand, you are spending 150 points on defence and filling up 3 of his ability slots with non magical things. Did you buy the Slann to be an unkillable block, or did you buy him to cast spells? Personally, when I use a Slann it will be to cast spells so all the ability slots I sued will be filled with magic enhansing abilities to get the most out of his already considerable points. So what if he is a bit more fragile, basically every wizard in the game is. From the middle of a unit of Temple Guard, very little is going to touch him and I can be happy that his points are well spent.
I have voiced my concern over a lone Slann before, I cannot see it as a good choice. Sure he is tough, very little will kill him, but opponents only need to win by 1 (ie. outnumber) to force him to make a break check, and there is no way he is going to win the combat on his own so he will get stuck in place by cheap hunters, which restricts his ability to use magic missiles. Also any opponent that has a lord on a dragon, or even hero on a flying mount, are going to mash him.