So far, the best I see is putting the slann on the side of the unit or in a bunker of saurus you don't really care about. I would love to make my Slann ghostly, but there is a large wood elf meta in my community and their arrows all count as magical ;(
Let's do math on that for fun! a unit of 30 trueflight arrows are facing backwards, 8 miles away, behind a mountain and drunk. So, naturally 20 hits on the slann. If the ghostslann is 3" away from an infantry unit, he gets a 4+ look out sir!, 10 hits, wounding on 5's. 3-4 wounds (I'll go with four) then you get your 4+ ward save, which means 2 wounds! I guess he wouldn't die, but I wouldn't risk it.