This boss, like the others from the Heart of Destruction Quest have two forms, this yellow one and the orange one, while it is yellow, it is very resistant to physical, a little resistant to elemental and holy and neutral against death (that's why people call him the "Sudden Death Boss"
While he is orange, he is more vulnerable to physical damage while still being resistant to elemental, meaning you can still use an elemental or physical weapon and hit him fine, but if you are more hardcore you should change weapons when he changes forms.
Yellow Form:
Sword: Tagralt Blade is probably the one that will hit the hardest, but you can still go for Gnome Sword or Summer/Winterblade if they are the only available.
Axe: Phantasmal Axe is probably the one that will hit better, but no problem if you go for Falcon Battleaxe or Cobra Axe, they will be fine too.
Club: Mortal Mace or Cobra Club, both will be the kinda' the same, Falcon Mace is also fine.
Orange Form:
Basically the strongest physical attack weapon you have (preferably imbued with powerful strike), here are just some sugestions
Sword: Sword of Destruction / Shiny Blade
Axe: Axe of Destruction / Crystalline Axe
Club: Mace of Destruction / Mycological Mace
Hope I've helped ^^