Yagluth is the first boss that you have to treat like a real boss fight. First of all, you need Frostner or a blackmetal sword and blackmetal shield, padded armor (all Level 4). Silver arrows work ok, but there's not much time to shoot. Serpent Stew, Lox Pie, and Bread for food. Have a Fire resist mead up at ALL TIMES. He does 3 attacks.

Flaming Meteors from the sky: Run away from him diagonally and hide under one of the altar fingers if you need to.

Fire Breath laser attack: Run closer to him diagonally, in the direction his breath attack is rotating towards.

Fist Slam: This attack produces a blue PBAE effect. you can just block or parry it. Be in melee range and preferably behind him when this happens (remember you just ran back closer to him). After blocking, unload damage on him with up to 3 3-hit combos. Be ready to run diagonally away when he calls back the Flaming Meteors.

Use health and stamina potions as needed.

Repeat.