Defeat all the enemies without getting hit or being captured by a Fault
Focus on the Chimera Seeds first then the Assaults with Pandora attacks. Once the Chimera Assaults come and if you have a full Disaster Gauge, use PF666: Omen to kill them all instantly. If any are left, take them out. Another way is to have a full Disaster Gauge and use Argument missiles and Omen as well. If you unlocked Super Dante, you can just double jump up and spam Argument, it will kill all the enemies before they have a chance to attack you.
Another way to kill the enemies easily is to use Coyote-A and Stinger to dispatch of the Assaults while dodging the Faults.
As Dante, this Secret Mission's seal can be found on Mission 15, when you go to the area where Nero got the Anima Mercury, head to the next room. You should see a Red Orb Crystal at the floor, jump down to the hole and the seal will be on the wall.