I'm going to assume you have the main mechanics down and are executing those correctly. This would be stuff like Force Leech taunts, Wither target healing (and staying spread out so it doesn't hit multiple people), Marked for Death kiting, and making sure to kill the proper add first. From the sound of your post, you aren't having any issues with these, which is good. Aside from those things, I've found that there are 3 very important factors to phase 3 on Nightmare.

The first (and most important) thing is knowing whether or not you have the DPS. This probably seems obvious, but even if you have not hit the enrage timer yet, knowing how good your DPS is will set the pace for the entire fight. If you're behind in one phase, each phase after will be more difficult. There are a few good indicators to know whether or not you are on pace. For phase 1, you want to aim for killing Heirad before the third Surging Chain. This will give you an excellent shot at cleanly beating the fight without seeing the enrage. If you can kill Heirad before the third Lightning Field you are still in the game, but you'll have to push hard. Failing to kill him before that means you're going to have a very, very hard time beating the fight. After that, you want to shoot for killing Ciphas around or before the 5 minute mark (any live parser or video capture should have the fight duration for you to check). Assuming no DPS deaths, if your DPS is consistent you'll kill Kel'sara just before enrage.

The second thing that most guides do not mention is that the 3 sets of add spawns happen precisely at 75%, 50%, and 25% of Kel'sara's health. This may not seem too important, but if you have any AoE's (XS Flyby's specifically) having them prepped and placed at the right time can really help you to melt the adds quickly, giving you more time to focus the boss and less risk of having anyone clumped up chaining the Wither debuff.

The third bit of help I can give is that the fight has a circle limit of 12 (counting BOTH green and red). Controlling these is easily the difference between a successful kill and an unsuccessful one. Although the initial placement of green circles in phase 2 is not extremely crucial, the placement of any that go out during the second half is. The way my group handles phase 2 (we also have a 2 melee + 2 ranged group comp) is having tanks kite Ciphas around the edges of the room as slowly as possible while having the rest of the group follow behind dropping green circles. If the tanks are roughly 10m away from the walls, melee should always have a free lane to run towards the wall to drop their green circles, and ranged should already be against the wall or near another good location. The ideal location for any green circle is halfway into the wall and slightly overlapping with a previous green circle. Try not to have anyone move ahead of the tanks, because that can cut off your kiting path. Similarly, try not to have anyone straggling too far behind because that can cause issues when Ciphas casts Strangle. If you have any Sages or Gunslingers that get Doom, have them immunity bubble or combat roll to avoid the damage at the end instead of picking up any green circles. The exception to this would be near the end of the phase; you want to make sure no circles are in the middle of the room. You should also remember that Doom will wear off naturally as soon as Ciphas dies, so whomever gets Doom around 10-15% should not need to worry about it at all, and also should not pick up any green circles. Depending on your group comp and luck, you should be able to have anywhere from 4 or 5 to 12 circles up. If you have multiple Gunslingers or Sages and you manage to have 12 green circles up going in to phase 3, you're all set. No red circles will drop unless someone eats a green one. If you do start dropping red circles, make sure that people are spread out and the middle of the room is clear. The *only* time someone should ever be in the middle of the room (where the adds spawn) are while melee are killing the large add or when someone is being targeted by Marked for Death (they will not be targeted by a circle during that time). It's also very important that melee are always prepared for Kel'sara's knock-back effect, and never have their backs to a circle.

I'm going to assume you have the main mechanics down and are executing those correctly. This would be stuff like Force Leech taunts, Wither target healing (and staying spread out so it doesn't hit multiple people), Marked for Death kiting, and making sure to kill the proper add first. From the sound of your post, you aren't having any issues with these, which is good. Aside from those things, I've found that there are 3 very important factors to phase 3 on Nightmare.

 

The first (and most important) thing is knowing whether or not you have the DPS. This probably seems obvious, but even if you have not hit the enrage timer yet, knowing how good your DPS is will set the pace for the entire fight. If you're behind in one phase, each phase after will be more difficult. There are a few good indicators to know whether or not you are on pace. For phase 1, you want to aim for killing Heirad before the third Surging Chain. This will give you an excellent shot at cleanly beating the fight without seeing the enrage. If you can kill Heirad before the third Lightning Field you are still in the game, but you'll have to push hard. Failing to kill him before that means you're going to have a very, very hard time beating the fight. After that, you want to shoot for killing Ciphas around or before the 5 minute mark (any live parser or video capture should have the fight duration for you to check). Assuming no DPS deaths, if your DPS is consistent you'll kill Kel'sara just before enrage.

 

The second thing that most guides do not mention is that the 3 sets of add spawns happen precisely at 75%, 50%, and 25% of Kel'sara's health. This may not seem too important, but if you have any AoE's (XS Flyby's specifically) having them prepped and placed at the right time can really help you to melt the adds quickly, giving you more time to focus the boss and less risk of having anyone clumped up chaining the Wither debuff.

 

The third bit of help I can give is that the fight has a circle limit of 12 (counting BOTH green and red). Controlling these is easily the difference between a successful kill and an unsuccessful one. Although the initial placement of green circles in phase 2 is not extremely crucial, the placement of any that go out during the second half is. The way my group handles phase 2 (we also have a 2 melee + 2 ranged group comp) is having tanks kite Ciphas around the edges of the room as slowly as possible while having the rest of the group follow behind dropping green circles. If the tanks are roughly 10m away from the walls, melee should always have a free lane to run towards the wall to drop their green circles, and ranged should already be against the wall or near another good location. The ideal location for any green circle is halfway into the wall and slightly overlapping with a previous green circle. Try not to have anyone move ahead of the tanks, because that can cut off your kiting path. Similarly, try not to have anyone straggling too far behind because that can cause issues when Ciphas casts Strangle. If you have any Sages or Gunslingers that get Doom, have them immunity bubble or combat roll to avoid the damage at the end instead of picking up any green circles. The exception to this would be near the end of the phase; you want to make sure no circles are in the middle of the room. You should also remember that Doom will wear off naturally as soon as Ciphas dies, so whomever gets Doom around 10-15% should not need to worry about it at all, and also should not pick up any green circles. Depending on your group comp and luck, you should be able to have anywhere from 4 or 5 to 12 circles up. If you have multiple Gunslingers or Sages and you manage to have 12 green circles up going in to phase 3, you're all set. No red circles will drop unless someone eats a green one. If you do start dropping red circles, make sure that people are spread out and the middle of the room is clear. The *only* time someone should ever be in the middle of the room (where the adds spawn) are while melee are killing the large add or when someone is being targeted by Marked for Death (they will not be targeted by a circle during that time). It's also very important that melee are always prepared for Kel'sara's knock-back effect, and never have their backs to a circle.

Wow, cheers man.