I've noticed these enemies are very troublesome to deal with, and can be a nightmare for people who don't know what to expect, especially the named captain, Eleane.

Here is how captains work.

When they do not have full mana, they will execute attacks that don't hurt too much. However, every now and then, they inflict a minor permanent DoT that will last for the rest of the battle or until your pet is killed. Also, if they land a critical hit, they are guaranteed to apply a DoT as well. You should first focus on making sure you have plenty of mana to use defensive abilities like block and other things.

Here's a very basic strategy which should work if you don't want things too complicated.

1. Attack the nightfall captain normally, but keep an eye on her mana. When it is one turn away from being filled up, that's your last chance to execute regular attacks, apply a taunt attack etc.
2. When her mana is completely filled up, she will use vanish on the next turn. Have all of your pets use quick attack so they generate 10 mana each and hit her before she disappears. It's vital your pets have at least 10 mana anyway for the next turn.
3. Vanish is extremely dangerous, and once it's on, you need to know how to handle it. Vanish basically forces all attacks to be critical hits, and it also grants immunity to being attacked. You need to focus on defence until vanish wears off.

The captain (including Eleane) will do the following three actions in sequence.

Backstab -> Heal self -> Moderate attack (can't remember exact name).

The Backstab attack *MUST* be blocked. This is by far the most important part of the fight. Have all your pets block if the captain is NOT taunted. If the captain is taunted, have the targeted pet block.

The next move will be the captain healing herself. You can skip attacks here. If you have any pets that can heal allies directly, and aren't too low on mana, then now would be the time to toss a heal at the tank.

The next move after that will be an attack that is guaranteed to critical, so blocking will help significantly reduce the damage, and negate the moderately nasty DoT that would be applied otherwise. It's unlikely that the captain will still be taunted by this point, so have all your pets block here, or skip their turn if they're on very high health and can afford to take some damage (if you want to conserve mana).

When you don't have to worry about vanish, just attack normally and keep a close eye on the captain's mana.

For Eleane, things are a bit more complicated. Eleane will mercilessly punish anyone who does not follow the mechanics outlined above (TLDR, block the backstab attack or you could take 8000+ damage from it total, including the DoT). With Eleane, you will have to take out the healers with her first, otherwise she's unkillable. Also, you need to react to vanish accordingly, and instead of skipping turns when you normally would, attack one of her allies. Try to get them down as fast as possible without suffering a critical hit from Backstab.

Once Eleane is on her own, treat her like you would a regular nightfall captain, with one exception. You need to be far more careful to follow the mechanics when fighting her. She's far less forgiving than a regular nightfall captain if you mess up and allow Backstab to land without being blocked.

You are mistaken. The heal is not every vanish. It's heal or [some attack that is not backstab], which will be a crit. I think the heal is every odd-numbered vanish? first, third, fifth, etc.

Oh, OK, thanks for the information.

Yep. The captain uses piercing wound during even rounds where he failed to land a kill. I presume eleane is the same, but haven't had a fight with her go on long enough yet.

That said, it's possible, if you've got a damage heavy party and build, that taking down eleane before the vicars is actually faster and/or more practical than the reverse. I nearly did that, but splash damage from my fire pet took out the vicars anyways. (Using almost 3 complete vinethorn sets, though, and rank 2 to rank 3 skills, so...)

Eleane also seems to never waste a turn on healing- she won't heal herself unless she's actually taken damage. I don't know how much she needs to have taken, however. If she's damaged, she'll heal on the first vanish, and the third, and the fifth- but there seems to be a turn of cooldown between mend chances- if she heals on the first vanish, she can't on the second. If she skips the first and heals on the second, she can't heal on the third, etc.