You can make peace with any race if you have 150% of his score and give a gift of 4000 minerals 3000 radioactives 3000 organics with a trade treaty as soon as you meet while the relation is moderate. The treaty Lasts even when the race turns murderous. Later in the game when you are no longer 150% and the AI is in the top five he will cancel the treaty. THEN you build a ship to guard your planet and blow up his ships and colonies in the same system. I find it far better to have about 39 colonies spread throughout the quadrant with peace with all AI until I reach score 500k. Then stay below 500k, scrap facilities replacing them with storage until you have the tech researched for maximum speed maximum shields maximum weapons quantum reactor minesweeping dreadnoughts, about turn 120. Typically I have stockpiled 5 million minerals, 1.5 million organics and radioactives and research 225k per turn with 30k intelligence. Do NOT build level 2,3 facilities or ANY computers as this usually *immediately* results in war. The war Lasts from turn 130 till about turn 150 and is very time consuming compared to the first 130 turns which take only about 20 hours, including half that time for the initial planning stage. An hour per turn moving 39 dreadnoughts and colonizing 150 planets plus managing the surrendered empires is typical. It is NOT necessary to blow up planets and is usually not necessary to invade with troops. Just blow up ships and blockade all mineral planets to get a surrender and use demanding tone on politics.

Of course the real trick is to be an Ancient Race so that you know exactly where the 39 biggest and best planets of your type are located, and to get that 150% of the first few races you meet....