To get the Forest Lord skill for Prier, you must first have the Forest Lord's child, obtainable by getting the Bad Ending for the Chapter 3. Go back after the chapter, to the last map of Meuniere Forest to get the Lord's child to join you. After getting the Lord's child, train him up to level 20, and then return to the last map of Meuniere Forest again, a cutscene will follow and the Lord's child will leave the party but in return, let's Prier gain the "Forest Lord" Skill which summons the Lord's child in battle, and he transforms to the size of the former Forest Lord and do some slashes.
This skill costs 60 sp to use but is a pretty decent skill. Note that you cannot lose the Child during training(happiness level 1) or you will not be able to get the skill forever. Also, you have to train him to level 20 BEFORE getting on Homard's Ship or it will be a long time before you can continue the procedure of getting the skill. This is because once you board Homard's ship, you will not be able to access the main land, therefore not being able to access Meuniere Forest.