So I'm still up due to hijinx, so we'll move on with Dungeon running advice. Finally. Where were we? Oh right.
Aggro Management AKA READ THIS UPDATE.
Aggro (Short for agression, IE a monster beginning to attack you, signified by TWO exclamation points above their head. One is just alert, and doesn't nessacerilly mean it will attack you, but we'll get to that later.) is a major and as a result very important part of a dungeon run. It's important to know what enemies will multiaggro (more then one attacking you), which will single aggro, and how aggro works in general. Monsters with less aggro on them (single) are generally less threatening then multiaggro monsters for this very reason.
Aggro is also the primary reason you bring more people. Multiaggro dungeons are made far far more possible by even just one more person. That's not to say they're impossible to solo, but you've gotta have a really good grip on the combat system (or be a ranged class that can one/two shot the multiaggro monsters) to stand a chance. As a result, generally, to be useful in a dungeon enviroment, you just have to hold aggro on your monster (or monsters) and hold down the fort on them, because you are eating their aggro slot, and as a result, if that particular monster multiaggros, nearby people do not have to worry about it and can focus on killing the one in front of them. You're also helping clear the room faster, and as a result causing people to lose less stamina/mana over the course of a dungeon. Very few monsters change aggro with any regularity, there's only one that comes to mind, Gargoyles, and as you'd expect they are horrible horrible monsters for exactly this reason. Luckily they are very limited to where they spawn.
So let's get to the actual aggro management. First, if you hit a monster, look for a !! above their head in a big white bubble. If it shows up, get some distance via knockback/knockdown usually, and then hit a defensive skill. If they're single aggro, counter or defense will do, if they're multiaggro, and you see another one coming at you at roughly the same speed, windmill is probably the better choice. If you hit both, you knock them both back and as a result buy more time for someone else to help, or generally just keep you from dying in a solo situation. The reason multiaggro is so bad is because your normal defensive skills, defense and counterattack, are very limited in their uses against them. ( If you can't use counterattack, Generally you want to counter first and then defend, because defense always begins loading immediately as you hit the button, but counterattack has to wait for animations to finish. So you counter the first enemy and immediately load defense and block the second.) In multiaggro situations, DO NOT BE A HERO. Don't rush out to revive people, you will pull more monsters to them and die, thereby making it harder for other people to revive them, and also tipping the odds in your favor. AGAIN, YOUR LIFE IS PRECIOUS IF YOU HAVE EVEN A BASIC UNDERSTANDING OF THE COMBAT SYSTEM. DON'T ASSUME YOU CAN JUST RUSH OUT AND TRY TO SAVE PEOPLE AND NOT CAUSE TROUBLE TO OTHERS WHEN YOU DIE. IF YOU ARE EATING AN AGGRO SLOT AND HOLDING OFF THE ENEMY, CONGRATULATIONS, YOU ARE HELPING IN AN IRREPLACEABLE WAY.
Anyway. If you see a single ! after you hit them, they are alerted, not aggroing (very few monsters do this, but some will), and you generally have time for a second attack after the first combo/smash. But don't dawdle, they'll prolly be on your sooner or later. Push the advantage and all that.
A note, if you are fighting an enemy that is currently aggroed on to you, and another monster acts like it's going to attack, but never does, that is called Semi Multi Aggro. What that means is the other enemy is aware of you, but not attacking. It will generally immediately attack you after you kill the first enemy, even though you didn't hit it. Watch for it.
Aggro speed is another factor, some monsters aggro slower then others. Animals will typically aggro slower then monsters (think goblins/kobolds), and generally the bigger and meaner a monster is, the faster it will aggro, although there are exceptions. Monsters with notable aggro speed are golems, for instance, and this is why you see people pulling the skeletons away from the golem and killing them, rather then running in and hoping they kill the skeleton first. Could they kill them in one support shotted bash? Probably. But there's no reason to take that risk.
So, you've killed your monster and other people are preoccupied with their own, what do you do? Feel free to help, but BE AWARE OF WHAT YOU'RE DOING TO THE AGGRO. Generally pulling an enemy away from someone is never a bad thing, but be aware that it can and often does happen. If you hit an enemy attacking another person, that enemy will lock onto you, usually, sometimes without a !! sign. If you're not sure, load a defensive skill. If it goes past you and attacks the guy it was attacking previously, well, he was probably ready for it anyway, and again YOU ARE A LOT MORE VALUABLE ALIVE THEN DEAD.
Iiiiii believe that is it. So that sums up most aggro quirks, and I will repeat again. YOU ARE A LOT MORE VALUABLE ALIVE THEN DEAD. DO NOT TAKE UNNECESSARY RISKS.