As long as the AB gets to the 80+ range, it should be able to hit anything reliably as most event server mobs cap out in the mid 70s-80s AC.
As DC scales a lot slower than AB and mob ACs, lower level casters should also be able to hit with this spell as long as it is properly calibrated to hit at the upper end.
Base DC is 60 for a CL 50 caster with Prodigy, Domineering, 50 in the appropriate stat, and no spell focus.
1.2x of that is 1d20 + 72, which is a tad low in my opinion.
1.35x of that is 1d20 + 81, which is fairly reliable.
1.5x of that is 1d20 + 90, which is also fairly reasonable.
I would say that - if we do want it to scale with DC - any number between 1.35x to 1.5x of DC would be fine, especially given that the damage at (1d6+1)/2 CL isn't exactly extreme.
Alternatively, it could be just equal DC, but be an attack vs Touch AC rather than full AC.
Similar coding/scaling could also be used for the Warlock's Chilling Tentacles invocation.