Easy Strength, Explosives, And Shooting Skill Advancement:

Once you get to the Shai-gen and you have three stars in agility go to the supply depot to the upper right of the top island in the Shai-gen section. Climb to the top of the next story on the building with the supply depot. use either the rocket launchers or the snipers and look at the other rooftops there will be guys on them and if not then go to another supply point and come back. then eventually you will notice that the shai-gen are mosty in groups on the rooftops thats where you get your strength up just beat them to death. be careful though they have riot shotguns, m-16s, and homing rockets.