It requires unique circumstances to be useful and unique circumstances to be possible to cast. I'm just curious if the stars every aligned for anyone to decide sacrificing a Slann in that fashion is a good move and then be proven right. You usually have to be winning to have Dominance and you usually have to be losing to havel the Great Leveler and you need to roll 35 to cast and you can only get +3 from High Magic being ascendant, a tall order. The scenario I see that can move the odds of using the Great Leveler successfully from neglible to slim is this. Play an army with a small number of very big units versus a very large enemy army. Take two Slann, one with Itxi Grubs to get a one time casting bonus. On turn one, use Magical Duel to temporary dislodge an enemy wizard thus gaining Dominance temporarily THEN cast the Great Leveler. Still be iffy on planning around using it....
Honestly, I wouldn't. I'd rather those two Slann be buffing up my units, as things get really crazy in SoM. It's a beautiful thing to see every attack from a Bloodthirster bounce off a T9 bound Carnisaur, only for it to then sneak in 6 wounds right after. Besides, dropping your massive 4kvs4k game to six units and two characters from the get-go kind of sucks the fun out of it.