DYNASTY WARRIORS 9 Empires

Achievement is cumulative over all playthroughs. Alliances that other Rulers suggested do not count.

If you still somehow missed out, like me, start up Gathering of Heroes. and pick 10 musou officers with high influence. Make sure you are the ruler.

Cao Cao (Influence on all Wei officers)
Sun Quan (Influence on all Wu officers)
Liu Bei (Influence on all Shu officers and Han sympathizers)
Yuan Shao (Influence on the entire Yuan family)
Zhang Jiao (Influence on all Yellow Turban followers)
Dong Zhuo (Influence on all douchebags)
Guan Yu (Influence on the Guan family)
Ma Chao (Influence on the Ma family)
Sima Yi (Influence on all Jin officers)
Meng Huo (Influence on all Nanman, which are quite a lot)

After starting, set full priority to domestic affairs. Now, suggest alliances to everyone else on the map. It's tedious, but eventually you'll get it done.