I have not tried but I believe the following can help:
1) Disable all NPCs but one so the AI will not have wars with NPC
2) Do NOT use extreme settings as AI loses troops in most fights
3) Remove all water from the map (choose pangea and then move water slider to 0 in advanced options)
4) Disable landmarks/teleports
5) Never build orbital sectors
6) Do not explore anomalies
7) Disable empire quests
8) Make all AIs allied vs you, or alternatively use a mix of AIs so stronger AIs can defeat weaker AIs