CORE send 2-4 stacks of buffed up autonoms (around 6 stacks of 2500+ on turn 100+) which appear from thin air near your borders closest to capital. You can leave 1 closest sector unoccupied and they will spawn here from now on. Then i switched side to empress Carminia in last mission she didnt wipe out enemy AIs at all and it looks like this works only then you are not on her side or some other requirement needed. From my 3 playthroughs of last mission neutral is simpler than Carminia side because she will reduce core players to dust given time and then Autonoms can be bought to neutral status.