Originally posted by EffTea:

Alas, looks like I'll have to fire arrows and hope I don't die, as is the prevailing strat. Thank you for your replies.

Use the pillars to block the projectile attack from the boss, relocate when it summons the vines.

You and your friend may have to try and focus your own Elder at the same time so that they don't switch aggro (hopefully) and be sure to fight during the day / clear ambient enemies like Trolls that may be lurking nearby.

100+ fire arrows for each person should do the trick depending on how good your bows are.