Tales of Vesperia: Definitive Edition

During the battle with Schwann, he will use a Mystic Arte called Blast Heart once you lower his health and he goes into Overlimit. This is your trigger to get ready to hit the boss.

After he uses Blast Heart, he will he staggered for a short bit. This is easy to see because he will physically stop moving and clutch his heart. You need to knock him over during this time period to unlock the secret mission.

One strategy is to try and stay away from him while he does his Mystic Arte, so you avoid the stun. Then go in with a long range attack to knock him down. I was able to do this with Azure Storm on my first try.

The other solution is to get close and cozy with him. You'll need the Recover skill and a Stun Resist accessory (see the clip below).

If you find the opening to be too short, just wait until he uses Blast Heart again. Each time he uses his Mystic Arte he takes less and less damage to break his shield to knock him down. By the third time he uses his Mystic Arte, you should be able to get it without trying too hard. I was immediately able to break his shield with a single Azure Storm after his third cast on normal mode during my first playthrough of the game with no grade modifiers. Just drag the battle on if you need too until you can get it. Set everyone on Defend if you're worried you're going to kill the boss.

You will see a short text congratulations with him and Great! when you complete the mission.



Late Edit: Here's the clip of me getting it on Hard on a New Game+. I had the Recover skill so I could immediately get back up from his pushback. I used the exact weapons you would have at the time you face this boss, so no Devils Arms tricks here. :)

If you're having trouble doing it at the time you fight Schwann, just redo the fight later once you have the proper accessories to try the fight again at Nam Cobanda Isle where you can replay bosses. It'll be MUCH easier to do, plus you can take a LOT more hits so you can really be close to him.