I found this achievement easy to do after the final battle and you are able to return to the open world. Once you defeat Grax, you have to defeat his leftover forces that consist of new Dreadnaughts. Just look at the Starmap and you will see a gauge on the right of the planets as to what level the alliance is. Once you visit a planet, you will be able to fast travel just by aiming towards it in spaceflight mode and pressing and holding the A button. So, if it is a new planet that you haven't been to, go to a nearby one and you can then jump to hyper speed after that. It doesn't take very long to get from planet to planet, just look out for traps. Once on a planet, find the excavators and the prime and destroy them, then go to the build sites and make an armory. You should be close to 25% by doing this. If not, liberate a few villages or observation posts. Then move on to the next planet.
There are only seven planets and three you had to get to 50% to build the tower to fight Grax, (one in each sector) so you only have 4 left:
Dark Sector (Easy)
Frontier Sector (Medium)
Trade Sector (Hard)
If this helped you, please give me a positive vote. If giving a negative, please let me know what I can do to fix this guide. Thanks for reading, and best of luck.