It's really hard to even beat this mission randomly when you can't really coordinate and luckily I just (finally) learn that Chat Macros actually meant shortcuts after pressing the Select/Window button on the X-Box controller. So now I can just select the shortcuts and give better instructions from the controller rather than typing it out, but back to the topic at hand.
I pretty much learned early on that I can't just keep running back and forth between towers like I did in the other missions because it'll cost me time, especially since now that there are smaller enemies that appear so they can kill themselves on the tower for massive damage. It's really hard to even say the right things in the heat of the moment, but for the most part the easiest strategy I can come up is trying to tell everyone to stay near the tower and don't come to the other ones unless there are no enemies attacking the towers. At first I didn't think it will be even possible to do it in the heat of battle, but now that I know I can just say things with a click of a button maybe coordination will not be so bad...
But that's only the easy part, the real hard part is trying to get the Super Hard Players to actually listen and pay attention to things like that, especially on the map, since that is more important than ever compared to the other missions. Well for now, I can only hope that taking the strategies that are posted here and putting into my Chat Macros will be good enough. Here's the ideas I managed to come up with I got from the story mode battle and this mission.
A. At least 4 people should handle a tower by themselves, if you see more than 4 people protecting one tower then you should go to another tower to protect it.
B. Stay near the tower you should be guarding and look at the map for more enemies that are heading your way. If there are none attacking your tower or about to, THEN you can go to the other tower, and help other players. Otherwise you're leaving a tower defenseless to be quickly destroyed by the self-destructing flies that you'll probably won't notice until it's too late.
C. Actually the most important strategy but your Priority Targets should be the Corrupted AIS's and the Red Towers, as well as the flies. Both the AIS and the towers shoot beams that will definitely destroy the towers in mere seconds if you don't put a stop to them fast enough.
D. Here's how to defeat the Corrupted AIS: 1st you can destroy their guns while they're getting ready to shoot it. 2nd. You can destroy both of the weak points at their ankles to stun them to permanently reveal a bigger weak point on their back.
E. Here's a big problem about the Corrupted AIS. As soon as they show up they'll get ready to shoot their beams almost immediately and they won't be marked any differently than the rest of the regular enemies, so you have to keep your eyes open. This is also why it's better to have at least 4 people on each tower, because if 1 person goes to confront the AIS, they'll leave the tower defenseless against the enemies that appear with it.
F. At least 3 people should be fighting against a boss or two, if you see at least 3 people fighting them, focus on defending the tower from the enemies that surround theme. Especially if you jump into a battle from another tower that isn't under attack at the moment and the 3 that's fighting the bosses aren't with one or more people fighting with them. (Keep in mind this is written with the other towers being attacked at the same time, if they're the only enemies there, then go all out.)
G. That being said don't focus on only the bosses, if the other Base Defense Missions haven't taught you that by that point, you need to know when to leave the fight to other people and protect the towers.
H. If you happen to be the only person on the AIS, focus on the enemies not on the bosses. Sure you can kill bosses faster but you can kill loads more enemies much faster and keep the towers safer than focusing on one boss. But at the very least you can still shoot the giant beam at them if you want to help people kill bosses much faster.
I. AIS has a harder time exploiting the Corrupted versions weaknesses than anyone else who wasn't in one. Like with the bosses don't put too much focus on them when you're in the AIS but at the same time at least destroy their gun if they're about to shoot beam.
J. In Story Mode, Apprentice was made to be destroyed by the AIS in Hardcore mode and I assume the same could be said for the actual raid itself. So it's very important to collect as many crystals as possible even if you're in the middle of battle, that way, you can use them at least twice during the raid.
Now am I going to use all of that in my chat Macros. No, I'm going to point out the important ones about grouping up, and then using them to ask for help with a tower. And that's it, I'm pretty sure there are proper strategies to complete the mission and that's kind of the point of this thread along with other kinds of discussions about it. But I hope these strategies and ideas will at least help some people out in completing this mission on the highest difficulties and I wish them luck in doing so.