Dynasty Warriors: Strikeforce

Experience is usually earned based on the deeds an officer earns in a battle. Deeds can be earned by defeating troops/officers. There are some orbs which will increase the deeds (which are converted in to experience) which you would want to get from the workshop in order to speed up the process.

Obviously the harder the level is that you play the more experience you will earn. After playing through each of the stories I was in the mid 40's I believe, and that was after obtaining a couple deeds orbs along the way.

The method Bog mentioned does work well however only for the earlier levels maybe 1-10 or 15. Personally I liked playing online because once you are a higher level and helping weaker people you get double exp for being the most valuable officer after each quest. If you are going for completion, you will have this achievement early on in your quest to become a dynasty warrior .

A easy way to level up on this game is as any character useing a bow will work well because on the first chapter there is a request called Bandit Trap and its easy because on the first level theres loads of people to kill and they keep spawning untill you get about 1000+ kills and the bonus if you want to do it is open the gate in 3 mins so just pass the first stage and kill all the people in the secend one then just go back to the first one and just keep killing good luck on getting this chevo ;)
after youve complated the game high difficulty bandit trap will come which gives even more and if ur settings are on hard

To unlock this achievement, reach level 50 (the maximum level) with one of your characters. You'll get this naturally while completing your second or third kingdom's story if you keep using the same character.
(After completing chapter 5 of one kingdom's story, you'll be able to choose the other two kingdoms for playing. You can use grown up characters of the first kingdom there.)