Observing Buffs and Debuffs -- Example
The Hall of Mirrors is a good example of an instance where looking before you leap gives you valuable information, and where being aware of buffs and corruptions on the mobs as well as debuffs on yourself is imperative if you are going to win.
Hall of Mirrors
If you stop on the ledge above the wargs and examine the situation, you can see five mobs: one elite and four normals. That does not seem to be very difficult for three players. Before leaping down, though, you should inspect the mobs.
You will notice a couple of buffs on the matron. You can mouse over the icons and read the tooltips.
She has a large armor buff and "a freezing aura surrounds this creature, coating it and its allies hides with a hard covering of ice." That sounds like she might be buffing the other wargs with the armor.
You examine one of the smaller wargs.
If you examine one you will see they have the same armor buff, and when the matron moves closer, a new buff appears. You can mouse over the new (red) icon and read that tooltip as well. The matron gives the normal wargs +200% damage when she is nearby.
So without having risked anything yet, you can see that fighting all five wargs together is not a great idea. You can either jump on the pair furthest from the door and try to take them out before she gets back -- making it so that the most you'll have at once is three -- or see if jumping on her in the middle of the room keeps either pair of sleeping wargs from joining in.
Shortly after combat with the matron begins, she howls and a corruption buff (identified by the bright blue border) appears. Once again, you can mouse over it and read the tooltip: "Matron's Concern. Resistance Corruption (60) The warg matron looks concerned about the well being of her pack. She will do anything to protect them. +90% mitigation vs melee duration 15 seconds. Applied on expiration if in combat duration 10 seconds. "
The make-up of your group will influence your decision here. If your DPS is a runekeeper or hunter, you might legitimately decide that the corruption can be safely ignored. A melee class will obviously want to clear it.
The last sentence of the tooltip is not clear until a corruption expires in combat and the borderless heart icon appears:
"Matron's Protection. Heals 908 every 3 seconds for 30 seconds." With no way to remove the buff, you'll have to wait it out. By the time the warg howls a second time and the corruption appears, you know the pattern necessary to beat this fight and the warg boss fight that follows: watch for corruptions and clear them before they become a 10k heal over time for the warg. Corruptions are also important when dealing with the taskmistresses and defilers later on.
You should also be reading the tooltips of debuffs that show up on you or your companions. The wargs can wound for 107 damage every 2 seconds for 20 seconds. The fear debuff is trivial in comparison. Once again, this is consistent with the theme of the instance, as the taskmistresses and Ergoth dish out wounds that can kill.
Paying close attention to buffs, debuffs, and corruptions the first time or two through a fight can be a challenge and may even result in a wipe as you give higher priority to detaching tooltips and scanning them than clearing corruptions and taking wound potions. Consider it an investment in future wipe prevention: your clearer understanding of the encounter will enable you to beat it every time instead of hoping you 'get lucky.'