I'm playing solo offline and after the changes I also found that Yagluth is probably the first real challenge of the game. You actually need a lot of healing potions now.

As others have mentioned you need the best possible gear and food and you have to tank his aoe damage to get your hits in. If he calls for meteors or does the attack from his mouth you just have to run. When you get down to one third of health or so hide behind a pillar and use a medium healing potion or wait for your health to get up naturally if your healing potions are on cool down. I think it took me at least around 20 minutes and a lot of healing potions to get him down finally.

AFAIK the silver sword is supposed to do the most damage to him if you can keep the debuff from the secondary attack up.