A quick solution to get the achievement Gekokujo.
Kill a 4WP unit or higher with Heimin
The Heimin has to do the last hit against an 4WP unit or higher. So go to the first level of the "Cutthroats" scenario, select an warband with at least one Heimin included and start it at easiest difficulty.
The following enemies show up: 2 Renegade units, 1 Kanabo Sanzoku unit.
Go try to bring him down to 1 or 2 health points and surround him.
Now play "Hand of Destiny" and "Enraged", so that the Heimin gets an extra dice and a missing one will hit. Repeat the level until you get the achievement.