I don't recommend the totem of prophecy either, since it is expensive and half the armies are not going to care about it either way. If you really wanted to just push people out or force leadership you could put the scar vet into the unit, or give the Slann Terror. On a personal note, I have had luck with the banner of dominion, since it makes Tzeench cry, if you are really concerned about magic getting through then take the becalming cogitation or the unfathomable presence. The war banner is just great, and even the banner of Huanchi is nice in a TG unit, since it can let you get some charges you wouldn't otherwise get against other infantry blocks. On a personal note, I never leave the slann without the cupped hands of the old ones, because miscasting is dumb and toads should never do it. Itxi grubs have also won me a game, when I was casting spirit of the forge and needed a few more, that spell wins games.
As an added note, I like to give skink priests one scroll and one power stone, because sometimes you roll the comet and with a power stone you can actually get it off, which ruins days and cancels out other peoples scrolls. The slann can also benefit from stones, or even hold the scroll you would have given to the skink if you aren't giving the slann anything else.
i personally alway take a oldblood on carny purley for the effect it has on my oponent. i general find use my hero slotts for magic espically with the new engine of gods, level 3 skink preist with more survialbilty than a slann yes please,. also alot of player rush to kill a slann were as they generally ignore skink priest, plus i always find people a wary of a oldblood on carny and spend way to much time trying to kill it, i found on may occassion having a oldblood has allowed the majority of my army to reach the enemy unharmed
Only once have my slann succubered to the lesser races, and that was when it was last man standing and got charged by 3 darkelf units in turn 6 of a 3k match! Terrible game of mine, first time I met DE... Also, if your opponents rush for your slann and ignore your skinks, which I think is kind of a strange strategy, I would go for a big TG block and boost your slann with lots of CR (warbanner,BSB e.c.) then lean back and watch your enemy try to bite that fruitcake