from my memory, his auto regen is almost at max lvl, and that was around 20 floors ago(lvl78, HP 14500, regen 600HP in 10 sec)
so now is floor 74, where the safe margin is 10 lvl above that, and kirito is always 5-6 levels ahead, lets say he's lvl90 with HP 20000 and regen of like 750HP(which is like around 3.5%HP regen per 10sec)

i doubt a floor boss would deal a damage less than 3.5% of his HP in 10 sec, i'll say at least 20%HP
so i'll say that the auto regen is there, slowly healing him, and might have been what just saved him

NEXT WEEK YUI!

Yay Yui!