Guerrilla actions are marked by green circles on your map, there are 104 in total. If you move your map cursor over them, it will tell you the specific task/tasks to accomplish. Some involve rescuing hostages, stealing vehicles, and destroying EDF structures. Here's a list of the types of missions that are considered 'Guerrilla Missions':
Also, if you get radio messages to hijack a convoy, help with an EDF assault or do an interception, these also count towards your Guerrilla actions total. You DO NOT need to finish these before you complete the game. After you finish the last mission you are able to go back and complete all of the actions you missed your first time through.
Finish 25 of these actions and the achievement will pop.