I don't know but at least in Gaia (most active server) it isn't result very difficult to obtain cortex (I am talking about to reach the first three places).
First of, you need to be with another four pilots that play well the reactor (full ancient and parsec/raven ships highly recommended). If you have a poenary in your team, he will need to distract the boss since wave 5 and above, while the rest kill all the enemies. Else with a chromian or slate or marlstone or any tank your squad will replace the poenary to take away the boss.
The second one, as the reactor is a challenge, it is more difficult to kill enemys instead of the regular ones on Sirius or Tau Ceti. So, your squad not only depends of what the ships you have to it and if you have a full ancient ships, also it depends on your skills.
You can use gravitons but if you do it well, you might not need to use it or at least not spend a lot (using it as a joker items)
I have seened in the international Discord, the pilot Livai sharing this score from server Trantor:
Using it as example, if one squad of three pilots can do it better than squads of five pilots, it looks like the rest of the people are lack of skill (or better ships/equipment).
I think that with practice you might go better. But it also depends of your mates too.
I seened in this event the most fairest reactor challenge. With 30 minutes of graviton cooldown, the people that always had used it each 5 minutes to buff and save themself, only can use it like it was created: for change the course of the battle instead of spam a lot of it to win always.
