This guide is tested in normal and hard, but should work also in brutal, cause the maths is the same, only the AI probably will bother you with more spy attacks.
The guide is based on a strong counterintelligence, for people who don?t like to get robbed all the time from enemy spies. If you like a more risky gameplay and if you don?t want to spend much money into agents then forget this guide. It is for people who like safety in their empire and it is meant for long gameplays in big galaxys with many stars and many opponents.
Ok, let?s start with the guide:
To stay safe and dominate the spy game I recommend 10 lvl10 agents in the long term (6 defenders and 4 attackers). The absolute minimum to run an efficient planetary intelligence agency are 5 lvl10 agents (3 defenders and 2 attackers). You have to accomplish this as soon as possible.
Do not explore too fast, especially in higher difficulties. 2 scouts with warp engines are enough for exploration, even in epic galaxies. As soon as you have contact, the spy game begins.
Once you start a new game, immediately recruit 3 agents and train them si?simultaneously?very 25 turns until you have the first contact. Now you should have enough money to recruit the next 2 agents. Now let always your 3 highest lvl agents defend your empire and train the other two. Alternatively let your highest spy assassinate the enemy spies of every new contact. But keep always at least 3 defending spies.
If a spy attack happens your 3 defending agents roll some dice, and the highest roll is counting. With 3 high lvl defenders you got a high chance to block the attack. But according to the maths the attacker has always an advantage, so double your chance for the highest possible defending roll by training up to 6 defending agents. In my experience 6 lvl10 defenders block almost all attacks.
The maths you find here:
StarDrive Agent Success and Failure Guide
To avoid training accidents and so the loss of expensive agents you need as soon as possible the +10 to defending rolls. With this a deadly training accident is impossible. To achieve this you need either the trait ?Duplicitous? (+10 to all rolls) or get as soon as possible the tech ?Xeno intelligence? (+5 attack/+10 defend).
So, according to your cash recruit and train new agents until you have got your desired planetary intelligence agency. Remember: have a minimum of 3 highlvl defenders all the time. Send up to 2 highlvl attackers all the time to assassinate enemy agents. If you feel the enemy spies weak and outnumbered, then start stealing techs. Always train the rest of your spy crew to achieve higher lvls.
To avoid death of expensive agents you can do the dirty trick: watch the turns in the diplomacy window and save before a mission is over. In case of casualties you may load the save.
In the mid game you should have achieved the goal of 10 lvl10 agents. From now on let always 6 defend and 4 assassinate and steal. You will now dominate the secret war.
Feel free to add your thoughts, comments and experiences, and if something changes through patches.
Fly save, reign wisely and have fun!