I suggest you don't, because on his own he is going to need some amount of protection while in a Temple Guard unit he needs basically nothing. Being a very expensive model and having his abilities quite expensive and limited, IMO you want as much into boosting his magic as possible. However you do save the points of a rather expensive Temple Guard unit by running him solo, so maybe it works out ok.
The main thing I would put on him first is ethereal, then he cannot be hit by cannons and the like. Also hunters/fast cavalry will have a harder time bashing him up, but probably still could just with static combat res.
Other than that, make him how you like him. That is about all the protection I would go for, make sure there is a unit or two of skink skirmishers near him for protection. Keep in mind that he won't get the LoS of a large target on his own, so you will either need to put him with your battle line or leave a gap for him to see through. A Slann should be able to cast every spell in a lore if he has the +1 dice per spell and all 6 spells, with that many spells running out it makes it tougher on your opponents defense but he needs to actually have targets for the one or two magic missiles.