Obviously you can cheat or cheese... but that's not my thing.

My approach (works every time):
1) Scout the area prior to summoning and clear out any mobs that will be drawn in by the battle/noise.
2) Fire resistance mead.
3) Summon the boss.
4) CHARGE HIM - Run to the sparkles as he spawns ... as soon as "poof" he's there, start wailing on him with Frostner.
5) As stamina or health wane, take meads, get in a few more hits.
6) EVADE - once heath starts to wane again and wait for the cooldown timers on the meads reset. This means playing "ring around the monolith" for about a minute or so. If he starts poking enough holes in the monolith that it is longer safe to hide behind, move to a different monolith.
7) CHARGE HIM - once cooldown timers reset - and repeat 5, 6, 7 as needed.

I use the same approach with Bonemass using the iron mace.

That said, I only play solo so I don't know how much harder he becomes when a second player is added or how having two targets affects his movement. Your mileage will no doubt vary.

Good luck!