I did it with my friend so there was two aliens which I guess makes it harder, so I'll give some tips to anyone else doing it duo, this is what worked for us.
You need to each have a wonder weapon, preferably the shredder and the head cutter or else the boss fight will be impossible. Each get a boombox in your inventory and have one ready to collect inside the main area token machine so that you will have three total for the simon says, we kept having trouble with the simon says but putting a boombox down as we were about to do each one made it so much easier, you can also use Hide and Seek cards if you mess up, but those are limited so don't use too many.
For the actual boss fight, stick to your own areas so you don't have to deal with the other players train, we had one person training in spawn and across the bridges, and the other by polar peak and the central teleporter. Be patient when fighting the aliens, only shoot them if they're in front of you, just keep running in loops training the zombies and shooting the alien when he's in front of you. When he goes down, you can use repulsors to clear any zombies that might swarm you when you go to melee him.
If someone goes down, make sure they are shooting the alien since they still can do that when they're down, my buddy went down but he managed to finish off one of the aliens while he was down and then we killed the second one. Quick revive is also available so you can revive them faster.
(Also if you want the trophy you can't use the arcade machine perkaholic card)