I kept him near a 4+ LoS in case someone wanted to try something smart on him.
About his targets, he's really only suited to combats he can win per auto. That means Single rank monstrous cavalry and the likes, as he has 2 CR and they have 1 (banner) and nothing else, he'll win - against the right monstrous cav. Now that's just the most brutal example offc.
I also consider it worthwhile to flank charge with him if you can, and if you're certain you'll win combat by it. 3 CR added, 4 if he also charged, that's a lot of CR your opponent cannot gather from him, in theory, it's free.
I am afraid of some armies however. Most Skaven shooting is magic, and i am testing him against skaven soon.
Mind, that most of this is theorycrafting, i only had 2 games on him, and 1 was a loss (not due to him though), in the other he killed a unit of squigs without taking a single wound, and ended the killing spree of a mangler squig (again, not a single wound).
i find it worth it, though expensive, in theory, you free up your TG points, in practice, i buy them anyways ;p
A, I don't employ saurus, but still need a hammerish anvil. Thus i go to TG, because i find them superior in every way (even without auto stubborn), though i have considered a CoC vet to sit them.
B, People might think your slann is regular, just don't want to make bad mid unit miscasts
C, They look awesome, damn they look awesome.
D, i almost have them painted
and E, They're a reliable way of killing armored units. -3 AS is very good i'd say (razor banner used), and the banner option makes them adaptable, meaning they can fit several roles.
Still toying with my Battle slann, but i'm quite certain i'd much rather play with him than a normal slann
Short answer : i play him passively, until i know he's safe, or i deploy him safe, so i can play him aggressively, the investment is too large to only be security, he's there to kill stuff! and heck if i lose him, i'll prolly be angry and moan for a sec or 2 (terrible loser, terrible winner too), and then i'll find it awesome that's he's just so hilarious!