I've been number crunching, and I believe I've approximated the formulas used to compute damage in the Epic Boss fights. These calculations come within 2 points of the actual damage for all of the hits I've recorded.

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Base Damage to boss by a level 100 knight = 246 * (my attack / boss armor) * advantage factor.
Advantage factor:
1 for single advantage element (i.e. Swamp Shaman vs. Jian)
1.33 for double advantage elements (i.e. AA armor vs Jian)
3 for Nemesis

This damage is then multiplied by 1, 1.25, 1.5, 1.75, or 2 depending on number of knights. Special attack gets an additional 1.5x multiplier. Some attacks will randomly get a miss (zero damage) or a critical hit (same as special attack).

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Damage done by the boss is more complicated; it depends on the boss's level, not just on its attack score. That's why the Jian at level 14 does a lot less damage than the last few week's bosses at level 60, even though they have the same attack/defense values. (Damage done by the knights may depend on the knight's level, too; I only have data from level 100 knights.)

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Base Damage to knight with no opposing elements = (boss attack / my armor) * magic number
Magic number:
Jian levels 1-19: 3 * (pseudolevel + 4)
Jian levels 20-?: (1.6 * pseudolevel) + 43 (tested up to level 37)
pseudolevel is the boss level except at the reward levels, where it's equal to the real level that is closest to the same attack/defense values. (level 15 = pseudolevel 19, level 21 = pseudolevel 24, level 28 = pseudolevel 31, level 35 = pseudolevel 39, etc. Level 60 = pseudolevel 67!)
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As with knight attacks, some boss attacks will randomly get a miss (zero damage) or a critical hit (1.5x damage).

I didn't start keeping detailed records of boss damage early enough to closely check this against previous bosses, but it matches for the ones that I did record.

I'm going to change my main knight's name to Theorycrafter now.