So 3 things to consider first, when building your group:
1. The trash mobs can be quite big so its not uncommon for the tank (if you are not using an cc/heal tank approach) to loose aggro of at least some of them. So everyone should b able to take at least a BIT punishment and dodge effects like overrun, fireline etc. and block heavy attacks. Dont blame the tank or the healer, you can block and dodge yourself
2. Tons of fire damage. If you are a vampire, bring fire res or stay back. If you are a ww, you wont like the spider boss.
3. Some of the bosses make melee somewhat harder, consider that and bring a backup range weapon, just in case. (as for the tank, bring the ranged undaunted taunt)

Things i will cover in here:
Bosses.

Things i wont cover:
Trash mobs
Quests
Loot
Achievments
Lore

So lets start:
The first Boss is a big spider in the second room (the one split by a small river). The boss has 2 healing adds, which should b taken care of.
The boss itself is pretty straight forward he does 3 things:
He can feed on the healers corpses like any spider would (breakable). He spits poison as a round damaging area on the ground, but he doesnt cast it, the spit holds longer and he can thereby cover large parts of the combat area in it. Staying out of the spit is kind of an natural idea. Be wary though, some reported the bug of not seeing the area, move with one that can in that case.
Last ability is a cone attack it does infrequently. Overall the boss is neither very dangerous nor hard to come by so i wont do any specific tactic discussion here.
Name of the mob is Mephalas Fang

Second boss is in the room of the late goblin warlord. She stands on the same spot as he did and jumps down for the ledge when u kill the last trash in her room, especially the ranged spiders can from time to time be a pain there. That can make for an instant pull, if you stand to close to where she lands (she jumps down towards the point where you entered the room, but she doesnt jump far).
She is an melee enemy and has essential 3,5 important skills:
1. She throws a projectile at 1 player (you can dodge by strifing out of the way). If that projectile hits, it will create a line between 2 players (sometimes it doesnt, but not sure if thats intended or bugged). As long as the line holds, it deals damage to both. You should break the line by running away from each other.
2. She splits into 4 shadowclones of herself. The shadowclones always start with full hp, no matter how damaged she is (although they dont have much hp). Each of the shadow clones can hit very hard with the heavy attack and onehit an unaware dd without a problem, so either cc em or block. If you need to revive someone, keep 1 shadow clone alive to revive and give the newly respawned time to regenerate some ressources. The shadowclone will dissipate eventually over time anyway, or when the revived is ready you can just kill it. Sometimes it occurs, that she does NOT automatically respawn. Just wait then, its a bug.
3. She summons 4 Shadowclones that toss chains at 1 player. That player is shackled and stunned at the ground with a sword slowly lowering to his head. You need to focus down 1 shadowclone , then all 4 dissipate. If you cant do it in time, the shackled one will die by having a sword driven through the skull. So try to make up who to focus before it happens. Like "always the north one" or "the one the tank/healer attacks first". Thing of 2 things here: The tank might be shackeld and loose aggro of her! And the 2 clone phases can come together!
3.5 Her heavy attack (if not blocked) knocks the target to the ground and kicks him back a bit. If that happens to the tank in combo with any of her clone phases it can make for some chaos, be prepared
All of her abilitys seem timetriggered, not HP triggered. Her name is Gamyne Bandu

Third boss:
A Firelady with 3 spider adds, stnading on the isle where the dreugh boss was before. Basically she cant do anything.
She will highlight 1 player at a time (glowing red) as a target for the spiders. She deals firedamage and here is the trick: if you kill any of her spiders, she will deal more! if you kill all 3 she might be able to oneshot your tank...
So just cc the spiders in place and kill her, afterwards take apart the spideradds, piece of cake.
Name is Ciironas the Shepherd

Fourth Boss:
A pretty big (but not pretty) spider daedra will be, where the endboss of the normal grotto was before. This one is a "bit" annoing. With 4 Essential skills:
1. She sometimes turns to a player and does an damaging , ranged, knockback/knockdown effect. Not sure if its dodgeable or blockable, info on this would b nice . The targeting here seems random.
2. There is an seeking laser effect on the ground which follows a player and explodes in the end. The explosion can deal a fair amount of damage but you can just outrun the laser without much efford (IF you arent knocked down at that time, which is a *** combo to behold^^)
3. She uses an AoE around herself. The AoE is rather average in damage but should be avoided nontheless. (this can be a pain for melee damage dealers). If knockback, aoe+laser hit a single player together, chance of dieing are...well..good :P
4. The point where she spawns (a portal betwhen the too big rocks) is also the point for her fourth ability. The portal will re appear and suck the closest player in. That player will then have to defeat 8 little spiders and can then come back. (dont send a ww :P) Some of the spiders are ranged, but most are melee. They react a bit strange, sometimes not moving at all, sometimes 3 attack at the beginning, sometimes 4, its a bit incnsistent. Anyway if you killed all 8, you can return through the portal, but you have to walk through. If somewhat seems that, if the same player enters the portal twice in a row, she tends to loose aggro randomly, so if you cant take her attacks (which arent very strong) you might want to consider to rotate your damage dealers through the portal phases.
Her name is Spawn of Mephala.

Fifth Boss:
The fifth is behind the shrine entrance, just upstairs from the fourth boss. The adds on both sides can and should be killed individually.
He has 2 major skills and a heavy attack that hits like a *** train^^
1. He swings his 2hand Wepon around himself, attacking a small area around his base, dealing high damage but he is NOT moving , so you can just get out.
2. He holds his weapon high in the air and a ray connects to each player. This ray deals damage to magicka AND health but NOT to stamina. It cant be blocked. Sorcs and physical dds are at a bit of an advantage here, naturally. But everyone should b able to pull though.
His name is Reggar Dark-Dawn.

Final Boss:
The Final boss is in the entrance room of the dungeon (do not forget to take the obsidian husk from Reggar with you, or it wont spawn). Notice that the fight starts immediatly if you have the quest and talk to the quest npc!
She has 2 signature Abilities and hits quite hard ranged with a spellstaff.
1. She will, after 5-10 sec randomly teleport to 3 different players and leave a black , expanding, damaging area on the floor. So standing all together is a wise idea, cause these area get quite big and do a lot of dmg. The black areas will despawn before she jumps again, but it can actually prevent any melee from actually damaging her! So bring ranged attacks.
2. She will raise her staff and blue flames will spring up around her. Soon afterwards , blue rays will hit each player. These rays can be avoided or cleansed by going into the barrier around the shrine. The rays can deal a massive amount of damage in a very short time (aprox. 3 sec) but the animation can hold longer than the damage effect! The effect can not be blocked and she cant be interrupted.
Her name is Vila Theran.

If i forgot anything or anyone has any suggestions, please let me know. I might do these for the other dungeons too, since i found it hard to find anything usefull on the forums to cover such topics. Most seem to be about bugs :P