Boss arenas are made for setting up. Get top quality food and check out the new healing potions. Make sure the arena is flat and free of obstacles. Your home base should be fairly decent at this point, keeping a permanent "well rested" buff will serve you well for the rest of the game. Bonus points for setting up a portal somewhere convenient, in case of the worst.

The Elder has two different patterns for range or melee. I generally find the melee pattern easier to contend with. Axe+shield is the name of the game. Just get in there, whack away, and get ready to flee when it summons the vines. Don't challenge the vines, just get away and heal up for the next melee opportunity.

The Elder's range attacks can be blocked by hiding behind arena pillars. Dodging helps, but be careful of your stamina budget. The bow strategy is all about lining up against the pillars and fleeing to a new location when the vines rise up.