When encountered in Altus, the Grave Warden is joined by some jars — take them out first before facing the Duelist.
The Grave Warden‘s moveset is pretty straightforward, which makes him not very challenging. The only quick attacks that the boss has are the small charge forward followed by a vertical attack, and the grab in front of him. Other than that, it’s mostly swiping attacks with both hammers. Apart from that, he also has one ranged attack, which is a medium-range low swipe followed by a leaping slam forward.
If you’re melee, try to roll to his left or right after each attack. He has both a 3-attack and 5-attack combo that both end the same way: you’ll know you’ve got a safe window to attack after he slams both hammers down on the ground at the same time. He also has a move where he turns in a half-circle then swings his weapon; once this attack is finished, attack. Until you learn his combos, it’s smart to just roll away over and over. Once you think you’ve got his moveset down, start rolling towards him instead and attacking after his combos finish.
If you’re slinging spells instead, you can kite backwards with rolls while he’s trying to hit you and launch a spell at the end of his combos. Look out for his mid-range swipe attack followed by a slam, and keep kiting him backwards while throwing a spell every now and then. You can also use summons here, which makes it easy to unleash several spells while the boss is distracted.