Nightmare is a pile of toxic mush with some garbage and probably one of the most challenging boss. Appears at the end of Mission 16, 18 and 20.
Slickford's explanation for Ifrit charges against nightmare
Every encounter has the same approach. Only difference is M20 where you don't have to throw 2 extra hits from Ifrit. Additionally, you can't set the light early before triggering the fight. You can also skip his final phase by Holy Water. The sequence is:
If Nightmare 1 or 3 Cycles did not go well then it's highly recommended to get sucked into nightmare space. Nightmare 2 is not ideal because of Griffon. DT Management is the key of this fight.
First Sequence Core 1
Second Sequence Core 2
Third Sequence Core 3
Final Sequence Core 4
The fight must end with back core not being broken.
Core 1 If you don't have the DT Runes at max then do a taunt followed up a nade shot twice. Walk slowly to the left till nightmare preps for Light Ray. Air Hike towards the core into DT HB. The first jump has to be raw.
Start doing Shoot > Stinger loop 8 times. The core should be on green during Core 1 sequence.
Once done, Taunt then quick switch to Ifrit as soon as you finish the cycle loop. Shoot the nade twice as you jump away from Nightmare.
Core 2 This part is usually where things get decided whether you dive into Nightmare space or not. If you could not setup the re-light well then you will have to consider doing that to not ruin Nightmare's core state. For example, the mini-nightmares spawns are random but they can be dealt with well timed-jump. Therefore, It's hard to see at certain camera angle to get the visual cue of fading light. It's the best indicator of when to re-light. Otherwise, you will have to count to 2 seconds.
Walk to Nightmare's side for spike deflect prep. It's always good idea to at least shoot the nade once to maintain high Runes.
DT shoot followed by 5 Full Charge Ifrit.
If Core 2 cycle has not done correctly (Missing last Ifrit hit or re-light fail) then dive into Nightmare Space.
Core 3 After the last Ifrit hit, switch to Alastor then do shoot roll till you are face to face Nightmare. This forces it to trigger the Light Ray again. You will need at least 4 DT Runes for this.
Do Shoot > Stinger loop again but this time should be 7 not 8. If Nightmare's health is not at this range then dive.
For the last piece Core 4, walk to Nightmare's side again for spike deflect pattern. End the fight with 5 Full Charge Ifrit.
The fight must end with cores at green state.
Core 1 Depending on what you did at movement section, one taunt should give you max Runes. Other than that, this sequence is extremely important. Note down the timing on when to jump. If you fail to do that then you won't be able to get the last hit. The timing for the last two full Ifrit charges are very tight.
Shoot > 4 Full Charge > 2 Normal hits > 2 Full Charge.
Before Core 2, you will need to setup re-light. Be ready to deal with mini-nightmares while trying to build back some runes. Walk to Nightmare's side for spike deflect prep.
Core 3 Immediately start building 5 Runes while staying at the same spot. This sequence is very important to get it right, so you could deal massive damage on Ifrit's finisher hit.
DT shoot > 5 Full Charge Ifrit > 3 Normal hits > Last hit Full Charge.
Core 4 Prep for spike deflect then finish the fight. If everything went right, this should be the last sequence you deal with otherwise you'd have to repeat this again one more. Diving into Nightmare Space is not good for this fight because Griffon 4 is arguably one of the worst fights in the game as it takes so much time to kill.
DT shoot followed by 5 Full Charge Ifrit
The speed of this fight depends on the re-light consistency.
Core 1 Try to build DT Runes as much as possible. Sometimes you may not have enough for the sixth hit. The jump timing is similar to the second fight. As a result, you will have to taunt twice after dpsing because you will need to keep Runes management high for the re-lighting sequence.
DT Shoot > 6 Full Charge Ifrit.
Core 2 Unlike previous encoutners, you need to make sure that you are close to the light mechanic. Meaning, you need to stay in the same line before you prep for spike deflect for few seconds. Most important part is to avoid getting stun-locked with nightmare shots.
Full Charge Ifrit. You can add 5th, but it can very tight to re-light and build back DT Runes
Core 3 Nightmare will try to move you away far from mechanic after that, so you will have to get on his left side to push him back. You will need to have at least 6 Runes.
DT Shoot > 5 Full Charge Ifrit.
Core 4 The last sequence can be rough if you didn't build 4 Runes back. When that happens you will either do 6th Full Charge or lose time by doing the light mechanic again.
Final Phase Breserk
Sequence can be 4 or 5 Full Charge Ifrit. The moment you see the breserk state, jump away and use untouchable.
Abuse Inferno till death. It's important to note that the reason why your input gets eaten away is due to Nightmare interrupting you with a stun. Therefore, you have to be careful from mashing it.
It's good to check the Frequently Asked Questions (FAQ) if you have general questions regarding the bosses.