Killing this boss makes the merchant disappear forever, so buy everything you might want from him beforehand

I encountered him when I teleported to Isolated Merchant Shack grace, it's not isolated to limgrave.

You do not have to come here from another Grace. You can rest at the grace outside, pass time until nightfall, then rest again to respawn the boss.

Killing this Boss does not remove the merchant, in fact the merchant will also now be here during the night. Also you don't need to travel here.
Just sit down at the site of grace, pass time to nightfall then walk a short distance (like pick up your runes if you already died once) sit down again and then you can fight him.

You can activate him from the nearby grace point. Just rest until nightfall, leave the grace point, sit by it again, and he is there.

Church of Vows is another location he shows up in. Gives 6000 runes and the Meat Peddler's Bell Bearing.

It doesnt make the warmaster disappear.

contrary to what the other guy and the reply says, bernahl will still be there during the day and night. they're not tied in any way

I've found that you can rest there at night to de-spawn the merchant to fight him