This achievement is obtained after going through the Underwater Palace and defeating Leonid the Dreadknight at the end.
Leonid isn't as tough as you would think but he has some devastating attacks you should look out for.
At the beginning of the fight he will surround himself with 6 chains that stand vertically, destroy these chains first, which will then cause Leonid to start moving. He doesn't move very fast as he just walks everywhere. Just pummel him with your fighters and make sure and use the "Y" healing command when needed.
These are some attacks to look out for:
XIIIth Word: He will stop and summon up chains from the ground right under each character, there is no way to dodge this except to be a decent distance away from him when he uses it.
Chain Combo: This is just his regular combo attack with his chain easily avoidable if you run behind him, beware not to get caught in the full combo as it get be damaging if he lands each hit.
ZERO Revolution: Very powerful attack, but the most easily dodged. Leonid will jump up in the air and start charging a ball of energy that he will throw at you. Simply run a short distance away then run around him in a circle, the ball will harmlessly fly right by you.
VIIth Violation: Leonid will walk slowly while flailing his chains around. Simply avoid him until the attack stops, then resume attacking. Your party will tend to still try and attack him while he is doing this so make sure that they are healed up.
Overall very easily avoided attacks so you shouldn't have to much of a problem. Also I believe you get a situation bonus if Capell doesn't land the final hit.
Note: The video also includes the 2 boss fights prior to Leonid: Kraken and Karathos