Mission 3 XP Farming:
Start Mission 3 (Trial By Fire), and set the combo bonus to XP by pressing D-pad Right. Play the mission until you reach the part where the ships begin crashing onto the shore. Fight enemies and try to get a high combo rating to increase your XP bonus. The higher the difficulty level, the more XP you can earn.
Eventually you will be tasked to take out the tower. Allow yourself to be killed by the archers as you attempt to do this, and you will restart at the beginning of the level where you can repeat the process over and over until you reach as high an XP level as desired to unlock upgrades.
Chapter 3: Ghosts:
At the start of the chapter, locate the three sets of skulls which arranged in
circles hidden along the beach before the chain tower. Have Marius interact with
them and they will disappear. After all are collected, a group of ghosts will
rush past Marius later in the chapter.