If your looking for easy skill ups, you can get some from Elemental Crests, go into practice mode, and use it twice for each character that can use it. It's easy because you only need to use it twice non-like other armour and weapons that requires you to use it at least 4 times. Use it, and end the battle, afterwards, feel free to drop it for better equips you acquire!
In Chapter 5, you will have a chance to acquire the key to hell that you may use later in chapter 8. To get the key to hell, make sure you have at least 1 TP, after you placed the "T symbol" into the "Blue Crystal." After you walk south twice, you will encounter a "black rock," make sure you examine it TWICE! You will discover a secret passage that leads you down to fight a demon dragon. It is weak against dragon equipments and holy weapons!