Hint #1

You have to keep hitting the Orc or you won't survive for long.
Now think carefully, there isn't a single hero that will constantly attack him.
But 2 heroes combined can attack him long enough if they work together.

Hint #2

Agatha and Reeve will ignore the orc as long as the Goblins are alive.
Oprix can only hit the Orc every other round.
Kurt will aim for the orc but will never land a killing blow on him.

Think this through, you can't kill the orc or the goblins fast enough.
So either way, you're likely to take a lot of damage in this battle.
Make sure your team can take the hits.

Hint #3

Notice the Orc will instantly kill Reeve if he is in play.
No matter what order or who his partner will be, nobody can defend him long enough to survive.
So you might think of not bringing him along this time.

Hint #4

Kurt aims at the orc by defeault. But notice his psychology heavily impacts his actions.
Can you properly navigate him to hit the orc for as long as needed?

Solution

As said before, Reeve is too fragile for this mission.
Oprix spreads his damage for different targets for the long game with his burn -
But you can't afford to wait all that time. So you can't bring him along either.

Only Agatha and Kurt are strong enough to survive this damage.

Now take a look at Kurt.
If you put him behind Agatha, his Codependency will force him to attack her targets. Because Agatha doesn't attack the Orc, that already puts you in trouble.
Even more so, if his target is about to die, he won't attack it at all because he is also Merciful

So your choice should be : Kurt, Agatha