"Master Combatant" PVP help please

Me and my girlfriend have been playing local coop and other than reaching hardcore level 50, this is the last trophy we need for the platinum 🏆

Could someone help us farm these kills please?

UK time if that helps.

I have heard some people saying you can do it on local coop, but it's always a vague statement, and we can't seem to leave our party to be able to face off. Was this feature removed or am I doing something wrong?

Thanks for any responses in advance 🙏