To get a higher rank in a battle, make sure that the final move you use in that battle is Ledah's Over Skill called "Lost Seraph". If done correctly, Ledah will say "Pay for your sins with death" and you should get a higher rank. For example, assume that there are three enemies. Kill two of them and have one remaining. Lower the final enemy's HP down enough for Ledah to finish him off with Lost Seraph. The battle will end and Ledah should say "Pay for your sins with death". You should now have a higher score and a rank.
In Chapter 1 "Angelic Advent" in Area 1-5, pull the sword out of the ground before jumping off of the cliff. This sword works especially well against any type of dragon/wyrm creatures.
In Chapter 2 "Pursing the Truth", in the prison second screen, enter Look mode. There should be an inscription on the side of one of the pillars. Read it and there should be writing that gives you a code. When you complete Chapter 2 and go back to Elendia, talk to Claude the Dryad in the first room of the magic guild. He will ask you to input the code (Left, Down, A, Up, B). Afterwards, he will thank you by giving you the Item Manual, which can be checked out in the "Extra Content" section of the main menu screen. The Item Manual tells you about all of the items that you have collected thus far, as well as allow you to see what skills each has to offer to every character.
Right after obtaining Serene's Pendant, go further into the tree. You will meet an injured Harpuia. You can choose to either kill or spare it. Spare it, and you will get her undying gratitude as well as "Happy's Plume". This item when used by Ein will do close to 200 damage to the enemy with the highest HP. Happy's Plume can be used ten times before it shatters.
After Cierra joins your team, go back to the branch you fell off of to reach the Pelga nests. In Look mode, check the shining object in the tree. Help Ein regain his balance and you will receive Serene's Pendant. You can give it to any of the girls to make them happy.