The best way to get this one is stick with either a starting character, or select a good, low cost leader. The one I used was a C rank Daigo Dojima as he has an ability which increases the deployment gauge by 150 each time it's used, very useful for getting the rest of the boys in.
You can make a lot of progress towards this as you work towards winning 100 clan raids for the 'On the Winning Team' Trophy.
To speed this up, you can put your selected character into the Chief Advisor role on the hierarchy screen, which will see them earn 2x EXP (this will be available by the time you have beaten the main set of missions).
The best mission for me was the 2nd from last bonus mission in Kamurocho (The **** Army! Kamurocho Part 2) as it is easy enough to beat quickly and also will give your chosen leader at least 17k EXP per run.
*EDIT* The same series of missions in Onomichi are also great for this. Part 3 is much more managable consistently than in Kamurocho, getting between 20-24k EXP if in the Chief Advisor Role per successful run.
If you don't want to continue with runs after the 100 wins, you can use Kiryu's personal EXP to level him up on the training section.
To unlock Daigo, speak to Masao (to the left of Joe) and enter the code: m5bxokqrsielv19d