The npc in front says that he is unarmored, so no piercing damage is needed. He is also not a ghost. If you check out the manual.pdf in the install folder you should be able to take it from there
the scarab lord takes time, patience and as much strength as you can get according to the stats listed in the manual pdf. a circle strafing also helped me, and backing off and focusing on dodging when you're getting low on health.
