How can I make the odds of actually being able to cast "The Great Leveller" as good as possible? I was thinking Slann with Focused Rumination + Wand Of Jet, while High Magic was in ascendance. But even then on average you'll get bang on 35 :/ Have I missed some amazing magic item? Of course, the Slann has to be as cheap as possible, and you can't IF a Cataclysm spell
Your best bet is to Summon a Slann with a Power Stone (after achieving 'Dominance'), not sacrificing one of your actual Slann you paid for (because that generally means a whole pile of awesome spells, an excellent caster+magical gear down the toilet). Of course, you'll have to spend power dice like a drunken sailor to make this happen (25+ to Summon, and only if you have 'Dominance' to allow it, then 35+ to cast 'Leveller), but if you can pull it off, opponents brains will melt from rage.
ascension, EoTG, whatever you can to get more dice, and for mythic item wyssan's weighted dice can make a low number higher, but its only useful if that number is rolled. unfortunately you cannot summon anything with magic items
Unfortunately the great leveller is so expensive it is just fluff. Anytime it is easy to cast you don't actually want to cast it. The best chance of it being useful is with extra power dice discipline, ixti grubs and when high magic is ascendant and probably with an EoTG also making High magic easier. This gives a target of 24 on 7 dice with an expected roll of 24.5. If you don't make 24 keep the ixti grubs for next time.
Perhaps I'm being narrow minded and not able to look at a wider picture... But I view "The Great Leveler" as an absolutely worthless spell and a bit of a slap to the face for Lizardmen. Sure the spell looks powerful on paper, but to want to cast the spell, you will be losing by enough margin to feel the need to cast it.... but in order to cast the spell, you have to have Dominance...which I find unlikely if you're losing that badly. All in all, I'm really not that impressed with SoM sadly.