It Was The Only Way To Be Sure
Win against 2 AIs after Nuking. Every. Single. Planet. In. The. Galaxy.
I have just finish a 10 planet game against two AI. I used the warhead on every planet, I tried to build stations but was unable (blocked) due to the nuclear blast...
So in the end It was only my home planet and the 2:nd AI home planet. After i destroyed it I got some other achievement but not this one. Is there something I have missed? any special requirements?
EDIT: Solved, You have to nuke your home world to get (just before you take the last AI home world)