Supposedly tz96 thorns on heralds should be 9% reflection (monthorns, with % reflected = slvl, and aura = [monlvl/10]), at least that's the only dataminer post I saw about it. Or maybe its 1+slvl and 10%.
If you're getting a monster to -50% dr from amp and hitting with 10000 phys at 2 fpa with whirlwind, you'd be reflecting about 17k phys per second and should easily kill a 7k hp 50% dr barb in one second of ww
% leech as an offset isn't reliable because too many monsters have 0% drain effectiveness like skeletons. Lifetap works, they'd have 50% dr and you'd deal 1/3 the dps and not die from thorns at all, unless it wears off or gets overwritten then whoops you're dead instantly