From Dark and Darker Wiki
With mostly common quality gear, Physical Damage Reduction build fighter, with Dark Cuirass as chest armor to get some magical resist, on normal run barely survives earthquake attack, if they miss timing of jump, however it takes them VERY low on health. I think that like week ago or something I was reading from somewhere in this wiki that damage would be 100 physical + 60 magical or something, but no idea how correct info that page had, or what page it was that had those numbers, it definitely looked different than this one now looks. But with around 40-45% PDR and maybe 18% Magical Damage Reduction that would mean around 100 * 0.57 = 57 physical & 60 * 0.82 = 49,2 magical, totaling into 106,2 damage through armor, meaning that 126 hp fighter build would be left with ~20hp, meaning 1/5 of their max hp from full health. To be honest that feels like LOT CLOSER to what my experience about week ago of being in Ghost King fight was. Since I was only one first timing that fight in our party, for considerable part of fight I remained at distance and used bow, so I was able to remain without any of those "take extra damage from all other attacks" stacks from pools, but I seemed to need to keep my health up at maximum, to ensure surviving earthquake attacks should I fail that jump timing (That I ended up failing more than 50% of time on that run).
Only slight "abnormality" in that run was, that we had two parties fighting it. Since our party ended up meeting other party that reached it at same time and we all just decided we would rather have the Ghost king fight than risk some extended pvp that leaves too little time/health/people alive to fight GK.