i have ~ 1k spelldmg (s2-s4 equip) with my 5 shamans,
i tested something today: bloodlust, 5x fire ele'sand than dps dps ...... after 29 sec the eventboss was dead

if you have bad equip you must use 1 earth and 1 fire ele's and 1 bloodlust to kill him (you must heal yourself and your ele's)

the boss hits my shamans (12.200armor) for ~1k-1,2k with normal melee hits (i have 11,4k life).

keep spamming lightning blots (2x lightning bolt ==> 1 chain lightning ==> Fireshock ==> 2 lightningbolt ==> chain lightning ==> frost/earthshock ==> ...)

if the boss casts his special attack move away quickly with your shamans (bind a key for walk backwards with all characters)
the special attack does ~3,0-3,5k dmg to my shamans.

stand near the boss so your chainlightnings will kill the adds (dont forget to move back if the boss casts his special attack).

if you cant "solo" the eventboss ask a tank or a healer to help you, so it would easier for you