[thejeff]

Take a look at the Magic Path Booster Guide, linked from the Strategy Index.

In short:
Spectres can get you up to S4. (S2D1 spectre + Crystal Coin & Skullcap. A S1E1D1 spectre can make the coin with Earth Boots and the Skullcap.)
If you've got Blood, one of the ArchDevils has S3, which gets you to 5 and lets you make the Rings, for S6.

The S4 Ether Lord would get you to 7, but getting him might be more expensive than just empowering.

Tartarians can get up to 7 in any path, but you'll have to summon and GoR a lot of them to get it.

Beyond that, you'll need to empower or use the artifact boosters.

As Gregstrom said, if you've got the gems to make use of Wish effectively, you've probably got lots of clams and empowering up part way won't be that bad.