Dragonblade
Adventurer
Tisan Balshareska (Ninth of Nine, Ruler of the Supreme Council, Arch-Geomancer, Sunderer of Dreams, She Who Turns The Wheel Of Fate). Female Human 20th level wizard/15th level priest (Grombar) and a whole hell of a lot of enchantments (geoglyph tattoos) laid on her. See the Ruined Kingdoms box set for Al-Qadim for details.
Interesting. I have that box set somewhere. Buried in my garage. I'll have to dig it out and look at it one of these days.
The other candidate that I can think of is:
Emperor Vulkaram the Dark from Spelljammer whose tyrannical rule extends across 12 Crystal Spheres (Solar Systems). I believe he is a Maedar Fighter of some ridiculous level and has an orb containing an entire trapped pantheon! See Under The Dark Fist for info on him and his legions of lycanthropic soldiers.
To be fair both would probably lose against Borys, but either would give him a hell of a fight. The one that would likely equal him though in power, and would probably win is:
I'm not familiar with this guy. I have some Spelljammer stuff but not this product.
Dregoth of Guistenal (City by the Silt Sea for Dark Sun), Level 29 Wizard/Level 29 Psionicist Dracolich Great Wyrm dragon. Not a god, but he sure wants to be!
Yeah I think Dregoth would be his toughest fight. He is a level lower than Borys, but he has that whole undead thing going on.