I just finished battling Yagluth solo, and it was an uphill battle.

A fuling camp on one side, 2 tar pits on the other, loxes on a hill and deathsquitos with the occasional fuling patrol roaming around.

I geared up with the Padded Armor Set, Blackmetal sword and shield, Draugr Fang with needle arrows, health and stamina meads, fire resist potion and the best food I could bring at the time.

Cleared out the camp and tar pits, then went back to base to sleep.

Started the fight, went better than I expected, kept the stone pillars between me and him whenever he breathed fire or summoned meteors.

Then the mobs came.

Biggest problems were the Growths and Deathsquitos.

The Fulings were a minor nuisance but kept me from focusing on Yagluth.

Anyways, since you are doing it with a friend, why don't you alternate keeping Yagluth busy while the other one takes care of the mobs that show up?

When you are both focusing on Yagluth and one of you gets to critical health, disengage while the other one keeps Yagluth's attention on him?

Or, make a dirt wall around the summon area to at least keep the ground mobs away?