The vid below shows the locations of all the Tomikichi:
some notes:
-You should wait at least five seconds after killing the Tomikichi as it might not register the kill, although it usually does register the kill imediately, it is better to be safe then sorry. Trust me I learnt the hard way
- You must KILL the Tomokichi, if they fall into water and die your going to have to restart the level as it doesn't count as a kill and won't add to the achievement.
- you can leave the mission as soon as you've killed the Tomikichi. Just return to the ninja village and select the next mission
- you do not have to stealth kill them, any type of kill is possible
- Tomikichis you've killed add up no matter the difficulty level. You can therefore do 20 in easy, 20 in medium and 10 in difficult if you want