Doing this with a friend, I believe he played the Arbiter while I used Johnson. At the very least, whoever is playing the UNSC leader should pick Johnson. His ultimate ability is passive, it allows him to build buildings for free. This is especially useful if you can continue to spam barricades wherever the enemies are coming from. If you'd like some general tips, take a look at my guide for the "Barricade" achievement, which you should also unlock if you don't have it yet when you get this one.
Arbiter should focus on building units and countering incoming forces. Johnson should focus on buildings and turrets. As you get further and further into the Firefight, Johnson should start lending Arbiter supplies as needed once the former has a decent army of his own. If at all possible, try to have at least one super unit each.
Waves are generated randomly, so there's no telling what you might run into at first. If your first wave is an onslaught of air units it might be best to restart, as these waves can make it difficult to properly prepare for the waves afterwards.