DYNASTY WARRIORS 9 Empires

You should start as a general under someone else's rule in order to get this achievement, if you are a ruler it won't be possible unless you have your kingdom taken over or relinquish your rule.

I got this achievement starting as Liu Bei during the Yellow Turban Rebellion under the rule of Gongsun Zan, I completed all of the war council tasks as much as possible and kept winning battles and eventually I was offered the role of Grand General.

The requirements aren't completely clear, but for the Strategist achievement I levelled up my ingenious (fan icon) as much as possible and was offered the role very quickly at around level 7 ingenious, so therefore I think the requirement for the Grand General role would probably be to have a high brave (sword) or commanding (flag) level whilst having a good relationship with the leader and fulfilling your war council tasks.

Recruitment levels up commanding and training levels up brave, fundraising and procuring rations also contributes to these two, secret plans will help a lot, so try to give yourself a title which boosts brave & commanding by a lot to make this go quicker. Just try to look for the flag and sword icon on tasks you need to do and you should eventually be offered the position.

According to the Koei Tecmo official manual for DW9: Empires "those with aspirations of becoming a Grand General should look at achieving the Invasion Objectives." so essentially focus on fighting and invading every month and I would think that having a good relationship with the ruler is still a good idea too.

Koei Tecmo official manual can be found here https://www.koeitecmoeurope.com/manual/dw9e/en/4200.html