8th Ed. How to use Life?

I think the importance of ToV depends on how your Slann is deployed. If he's not in a unit, then the miscast isn't terrible with the exception of 2-4 & 10-12. However, if it's in Temple Guard, then a roll of 5-6 or 7 on a miscast becomes terrible, meaning you're hoping for an 8-9.

As an aside, the first 'Ard Boyz I played in for 8th, I cast a ToV boosted Flesh to Stone on a large block of Swordmasters (I was playing HE) in a match against a Skaven list with a Screaming Bell. They rolled an 11 with the Bell on 3 dice which among other things does D3 wounds to all T7+ models on the battlefield (not units... models) and wiped out the unit. I was aware of the possibility, as I play Skaven as well, but it was a rude awakening to see it in practice. So, if you see a Bell, be wary of using this combo until it is gone, because 11 & 12 are very common rolls using 3 dice to ring it.