Yagluth has pierce resistance.
as Nascarman said the best weapon for it is a Frostner or other bludgeon weapon.

Some tips that I used when I fought hime, I dug a trench between and under the "finger" pillares and put a portal under each one connected back to a base. Have a chest with extra fire resist potions near the portals in the safe area.

The portals gave me a quick egress to recover HP and stam should plus the ability to get from one side of the area to the other quickly and made it easy to do a dash and grab on my marker when i got killed.

empty you inventory of everything that you will not need. reduces the chance of not getting your stuff from a marker becasue your too heavy or the bags are filled with crap you picked up during the fight. (stone, wood, black metal scrap, etc.)

The trench also gave me some cover from Yag's meteor attack and made it harder for any adds that joined in to flank me.

Be patient and don't rush the fight, its going to take awhile regardless of the weapon.
When your HP gets low retreat, even if he has the tincy tinyst sliver of health left and FULLY recover. big mistake I always make in a boss fight is rushing back in before my HP is at cap again.

and lastly, be sure to get the best foods you can and eat them just before you start the fight, the HP/stam fade in this game ticks me off but that is how it.

Good luck.