You'll first enter the Rift Planes: The Mirage in Episode 10 - however, it isn't recommended you fight the Nevi until you are stronger, as you likely won't be able to defeat it at this stage. Also, due to Dusty being weakened, you'll barely even have enough energy to fly to the Nevi when you first come here, let alone fight it. In order to reach the rare Nevi, head to the third flower and look out into the distance, where you'll see a red, planet-like orb being suspended.
Once you reach it, try get close to it while dodging the red orbs it fires at you. It will cease firing momentarily between barrages, which is the only opportunity you have to attack safely. Use your Gravity Kick to kick the eyes on its back, then keep dodging when it starts firing red orbs again. If you run low on health or need more energy, retreat to stock up - the health gem by the first flower will restock infinitely after opening the flower - then head back into the fight. It doesn't heal at all, so take your time. Don't bother using special attacks to hurt it, as it is immune to special attacks.