The Last Story/Gurak Military Base

Presuming you have done Yurick's and Mirania's important missions, you should head to where the person has fixed the door. Use seek to look at the book. Zael and Calista will have a conversation. You acquire a key to where you fought the captain's monster. Look directly left with the seeker. Use the seeker again to look up on the map. Make sure you have upgraded and maybe buy new weapons for your crew, as well as armour. Sell anything that you don't need before you head to your group that's hanging in the captain's area where the map is for your next mission.

Easy does it
Swim to the nearby spot. Once you see an enemy sentry station on another area, use your crossbow and he'll fall into the sea. The next part involve ogres. Knock the leader with your crossbow to draw attention and knock him with a slash. Use your powers to get the monsters to gather to you to save your teammates from ogre pain. It's time consuming but not that difficult to deal with.

Outside the island
You'll approach Zangurak's Island but you have to go directly on the right. There'll be a group so take out the leaders and just knock everyone down, along with your powers to keep your team from dying. Once that's over you'll approach a save point. Save at that point should you lose progress as it might be difficult up ahead.