Wouldn't work with the sneak attack dice, but I have a better idea in any case. To get Overchannel in the first place, you need to be a 14th level Evocation Wizard. The 5th-level spell (You can get it at 9th-or-10th-level) called Steel Wind Strike deals 6d10 force damage on a hit, with you being able to make a spell attack against up to 5 targets. Overchannel that and every hit is 60 force damage, but if you have the Assassinate feature as well from Assassin Rogue and get any amount of hits against surprised creatures, then the dice should double from it automatically being considered a critical hit. Therefore the maximum damage should double too, dealing 120 force damage per hit. I think that's probably the best you'll get out of it, just make sure you have a high Intelligence since they're spell attacks, not weapon attacks.

Wouldn't work with the sneak attack dice, but I have a better idea in any case. To get Overchannel in the first place, you need to be a 14th level Evocation Wizard. The 5th-level spell (You can get it at 9th-or-10th-level) called Steel Wind Strike deals 6d10 force damage on a hit, with you being able to make a spell attack against up to 5 targets. Overchannel that and every hit is 60 force damage, but if you have the Assassinate feature as well from Assassin Rogue and get any amount of hits against surprised creatures, then the dice should double from it automatically being considered a critical hit. Therefore the maximum damage should double too, dealing 120 force damage per hit. I think that's probably the best you'll get out of it, just make sure you have a high Intelligence since they're spell attacks, not weapon attacks.

Or even worse, upcast Scorching Ray with the same method. If all attacks hit, 144 (+Int Mod) damage on a single target. Still relies on surprise but that is MAXIMUM POWER.