Not impressed with most tactics presented so far unfortunately*, they either tend to rely on your opponent agreeing that the Alignment is not magical (Why would you say it isn't by RAW? It's an attack in the magic phase- while nothing states it is magical, nor does the book bother to tell us if magic weapons give magic attacks. RAW has its limits, and common sense is sometimes required to fill in the gaps), or extremely good dice. Wraiths and Banshee are not a great choice, unless you have 5 or more in the unit, as the Priest will just challenge the Banshee every turn otherwise, and will be unlikely to take a wound. Scream it to death? Most likely LD9 if close to a Slann, and why wouldn't it be? I like the Forbidden Lore/ Bear's Anger idea but it basically involves you getting a spell past Lizardmen's formidable magic defences when using 2 casting dice- and a list WITHOUT a Slann should still have at least 6DD and 3 scrolls (unless your opponent is going easy on you). If you do kill the priest on top - which happens, it only takes a couple of bad rolls - then I'm not quite sure why a monster reaction test would ever send the Stegadon off the board either...
*Not meant as an affront to any poster here, it just goes to show that the tactics are very limited and do rely on good fortune, which is the hallmark of a poorly thought-out unit, like the Steam Tank, War Altar or Anvil of Doom. I'd like to try whacking the Priest with Black Knights, the enemy is likely to not try and stop you from getting close with them since it knows it can burninate them otherwise. However, a smart Lizardmen player will just protect his Engine with a small unit of skinks to the front, meaning that any attacking unit will only get to fight after it has first suffered a burning alignment...