Nevan is encountered as the level boss of Mission 9. Her fight approaches determined on the cycles, which are usually done in 2. However, there is a very rare RNG attack pattern where she would skip electric floor.
This boss in Mission 18 is optional.
Dante have to make her fall down by either Aerial Cross with A&R or Shotgun. He also needs to build Rage-meter during her electric floor or her shadow globe after you DPS.
There is a rare case where you won't be able to guard the electric floor. This affect the quick kill a lot but, it's still possible to salvage this with good Aerial Cross JC's, IF you managed to guard her shadow globe.
Using Shotgun means you will absolutely need to make 1.5 Rage-meter for the next cycle to kill Nevan. Although, if the first cycle was on 1.5 Rage-meter then you will need 2.0.
Vergil follows the same steps minus the RG special. He can potentially break her shields with 1 Starfall. He also needs to avoid her fall down. Because, it will ruin his DPS.
The same as NG Normal except she does more varied attacks.
Dante usually starts with 1.5 Rage-meter. Worth noting that Flux Release granting more damage than it usually does can happen.
With Vergil, it's very extremely strict. Missing JC's can cost 2 cycles most of the times. The position can play big factor of it's consistency along with PS hit stun.