Mortal Combat is the last mission of the New Frontier Quest where two players fight spawning monsters on the arena. If you have completed that mission successfully one time, can you enter the arena again with another player to help them complete the mission?
No, you cannot. This is part of what makes the mission hard, you can't have a higher level friend that has completed it helping you out. It's the same concept as other quests:
Fansite item design contestDecember 31, 2018
Introducing Review QueuesFebruary 11, 2019
Why do we allow answering own questions?February 11, 2019
Golden Post of FebruaryJanuary 28, 2019