In this secret mission, Dante must perform Royal Block five times in a row.
This mission is a test of patience more than anything. First of all, kill one Mephisto. Then you can concentrate more on the remaining Mephisto's attacks. Watch its movements and execute Royal Blocks. Destroy it and then get Blue Orb Fragment. It's actually quite easy to get all five by simply making sure you always face the lone Mephisto and rapidly pressing the Style button. However, the player is required to have at least a little experience on using Royalguard.
This mission does not require you to perform a Royal Block on five attacks in a row, just five Royal Blocks in a row. Dodging or otherwise avoiding the Mephisto's attacks will not break your streak; only taking damage or performing a normal block will reset your counter, with one exception (explained below.)
The easiest Mephisto attack to block is the extending finger. Listen carefully to the attack. The Mephisto's finger will glow while he charges the attack. There is a distinct audio cue which rises in pitch. When the cue has reached the highest pitch, press the block button and you should execute a Royal Block. After a few false starts you should be able to pull this secret mission off. There is no time limit so don't get frustrated and don't give up.
The Mephisto's spinning cape attack is also simple to block. The Mephisto will spin around, lifting it's tail and fanning its cloak as it does. Just before it scythes you with its tail, it will pause. Wait for that moment, then press the style button. It's imperative to wait until you see the Mephisto pause, as preempting the pause will cause you to block too early.
The charging attack is, perhaps, the most difficult to time. The Mephisto will leap back a great distance. Depending on your camera angle and position within the room, it may end up out of sight, either behind a wall or behind the camera. If this happens, just double jump away to avoid ruining your streak. If you can still see it, wait for it to extend its claws and drop into the ground. Once it does this, wait a split second before blocking. As with the spinning cape attack, if you pre-empt the drop, you will block too soon.
If you're still having trouble getting five in a row, there is a trick to help give you some wiggle-room, or at least take the pressure off so you can concentrate. First, you need to get the Royalguard style to level 4. This gives you access to the Dreadnaught form. Then, you need to get some charge on the Royal Gauge, either inside the Secret Mission or outside. After you Royal Block several times in a row, however many you can usually get before tripping yourself, activate Dreadnaught. While in this form, even if you take damage, it will not count as a "normal" block, and thus your counter will not go down.