Played it today against an empire army and it worked really really well. The Slann was able to avoid getting charged (until the very end) and could zip around buffing and magic missileing where applicable. Although as luck would have it, the 2x skink priests I included both miscast on 2 dice and 3 dice respectively each in the same round killing at least 4 temple Guard each which seemed to land me in the same boat as before! But even then, the only artillery my opponent fielded was a steam tank which chipped away at saurus warriors the whole game. My downfall was his deathball of 20+ knights that I couldn’t scratch. Swapped out spells for metal but they did nothing. Best casualties they suffered were from standing in a boiling river. No idea how to deal with them!
No worries. I assumed you'd meant that it had to be a 'normal' slann (so a giant spell casting toad is normal?) but thought it best to clarify in case another reader tried to add an ethereal to a unit. All good.
I have just had a though - would giving the slann magic resist give him a 2+ against magical attacks? and would this count for skaven artillery for example?
Unfortunately that won't work. Magic resistance only works against magic spells. It has no effect agaisnt magical attacks (otherwise it would be an auto-win against a Daemons of Chaos army).
I like ethereal Slann, but I think making the Slann flying would be overkill. Even ethereal my lone Slann tend to hang out behind my main lines. That said, against the Empire, maybe a flying ethereal Slann can tie up the Steam Tank.... My most common opponent for 8th, since Age of Sigmar came out, my only opponent for 8th edition usually plays Dwarfs or Chaos so Ethereal Slann would be near useless for me now, but I did have fun playing Ethereal Slann against my friend who moved and played Orcs and Goblins most of the time. My favorite time was when I charged into the flank of some trolls with the Cascading Fire Cloak. My plan was to wear them down with fire slowly, but it turned out I won combat by one with a flank charge and they broke. Ran down by a Slann...
You are absolutely correct! Illegal build. I should pay attention better. I knew there was a reason that I never fielded such a build.