The easiest, and least frustrating, way to get this achievement is to play just using Capell in a party by himself. This way there is no need to keep track of your stupid AI teammates, sheath your weapons, scream at your TV, throw your controller, etc. Here's the set-up you need:
- Make sure you are playing on Easy or Normal difficulty. (I was playing on Normal).
- Make sure you are at, or near, level 30 with Capell by the time you reach Port Zala. (I was at level 32). If you need to level up, go to Bihar Mines and fight the Lumpur Leaders.
- Have Edward smith you a decent sword. I used the Crescent Sword.
- Have Edward smith you the Mercurius Armor. This gives you a -50% water damage. Go ahead and have him smith you the Mercurius Headgear and Mercurius Greaves if you want, too. They're not necessary, but they add a good armor boost. The armor is the important one.
- Equip the Ring of Life, if you have it, for health regeneration.
- Go to the Party menu and remove everyone except Capell from the party. Go to options and set cursor memory to ON. Go to the Items menu and place the selector on your best healing potion (I used Green Berry Potion) although using this strategy I never had to heal myself even once during the battle. Just set it up for safety.
- Go to Battle Skills and set Eternal Refrain to your "B" button.
- Make sure that you have at least 1 Sandstone, 1 Ceramic, and 1 Cork Wood in your inventory.
- Save your game at the Inn.
Okay, now you're ready for battle. Here's the tips you need for the battle itself. After the first cutscene, and before the battle starts, go to the Enchant menu.
- Enchant Capell's weapon with "Earth Damage", then enchant it again with "Earth Damage +30%".
Head on in to the battle. Start off by taking out all the crabs and fish, then hit the boss with Eternal Refrain. This will usually down him in one hit. If not, just hit him a few more times with normal attacks. Once he's down, go pound away on the chain. When the water starts to recede, go take cover.
Be sure to take cover behind the V-shaped wall in the middle of the beach area. This covers you from the front and both sides. It will usually even cover you if the boss comes around behind there and knocks you down. Watch for the water receding as your cue to retreat to cover. Do not wait for the controller to vibrate before running for cover. You won't have enough time.
Repeat the sequence of taking out minor enemies, down the boss, pound the chain, take cover. You should probably kill the boss on the third time you attack him, maybe the fourth. When he's dead, just go hit the chain one more time to unlock your achievement.
When the tsunami cutscene is ending, if you see yourself laying down and being swept along by water, then you are going to need to reload your save and try again.