After completing Taejin’s Tower in Chapter 11 you can begin using Mission 24 - A Potent Sting to farm both Crystogen Points (CP) and Gil. The prerequisites for being able to complete this Mission are that you need to have completed the following other Missions:
Mission 21: A Tremulous Terror
Gelatitan located on the Second Tier of Taejin’s Tower.
Mission 22: Infernal Machine
Ambling Bellows located on the Second Tier of Taejin’s Tower.
Mission 23: Natural Defenses
Gurangatch located on the Second Tier of Taejin’s Tower.
But you will have already completed these Missions if you completed the Taejin’s Tower section of the game as each of them are required to be completed before you can reach the Apex of the tower.
Travel back to Taejin’s Tower, which can be reached easiest from the Waypoint in Oerba. Just follow the lengthy trail out of the Village, through the mountains, until you reach the elevator at the end of the path which you can use to travel back to the Apex of the tower. The enemies along this path are very easy to dodge.
Make sure when you get to the top of the tower to take the middle elevator back down into the Tower as it can reach every tier. Use it to travel to the Fourth Tier and then use the other elevator on that floor to get up to the Fifth Tier where you can find the Menhirrim for Mission 25 nearby.
So why farm this particular Mission? It’s simple really. The mark for the Mission is extremely close to the Menhirrim that gives you the Mission, which means you can repeat the Mission over and over and over again.
There is no real strategy required for the fight as it’s pretty easy and it gets even easier if you come back to farm it after completing the last portions of Chapter 11 in Oerba.
The rewards for completing the mission and for each successful reattempt include 6,000 Crystogen Points (CP) and a 
Moonblossom Seed which can be sold for 6,000 Gil.
