This can also be easy or hard depending on your team. It relies on everyone staying alive. Atheon will transport the three people farthest from him to one of the two gates. It is essential you make sure it's the same three people so that you are all in the same rhythm. I'll break the final boss into two sections for those of you that will be in different spots.
People Staying outside the Gate
Your main goal is to stay alive. Atheon will spawn exploding harpies called 'Supplicants' Your entire goal is to not get blown the feck up and to make sure that the gates get opened for the people that get transported. Stay high and out of the reach of the supplicants. Once the gate is open make sure that you focus on taking out as many supplicants as possible
People Getting Teleported
The person with the most experience with the shield should be in the back corner, along with at least one hunter and the weakest of your group. There are different strategies for both planets. So I'll list them in different sections. REMEMBER TO CALL OUT WHAT PLANET YOU GET TELEPORTED TO SO YOUR TEAM CAN OPEN THE GATE
Mars
Mars is quite simply the easiest planet to get taken to. The person with the shield should be able to take out the three hobgoblins that spawn without a problem. One slam attack should be able to take out all three, and even if it doesnt it's doesnt' take long to do another one to take out stragglers. ( Dont' get me wrong. Sometimes they are spaced too far apart and its more difficult but this rarely happens )
The remaining two people will need to focus on shooting the oracles as they appear. Make sure you have a rapid rate of fire weapon equipped with good stability and a large clip. When you get the last oracle make sure you call it out and you'll get additional 10 seconds before Atheon starts walking around.
Once you are out of the gate DON'T STAND STILL TO BE CLEANSED. Your team should run to the back spawn point and get cleansed there. That way you have less of a chance to be outed by Supplicants.
Venus
Venus is a bit trickier since there is a Praetorian in there along side two goblins. This is why you bring a Hunter with gunslinger equipped. If it's the first try he wont' have his special up, so you will need to work as a team to take out the Praetorian. If its your second+ time through he will already be super charged and he will be able to use his golden gun to take a fair amount of health off of the Praetorian before the shield-wielder gets down to him. Be safe, be smart and Venus will be as easy as Mars.
Once everyone is out of the Gate
Titans should be equipping their shock grenades. Grenades do the most damage and you are given infinite grenades and double damage for 30 seconds. Shock Grenades for titans do nearly 4,000 damage per shock, and it'll land 3 shocks if your lucky doing 12k damage per grenade thrown. Having six people spam grenades like its a PvP match will drop his health quickly and result in less trips through gates.
This is a simple rinse and repeat method. Get in sync, get in a rhythm and Atheon will fall.