You only need to do 5. Out of interest, which have you done by accident?
Assembly of Iron either Medium or Hard mode gives you a Archivum Data disk QuestItem (one per kill). This unlocks 4 quests when handed into BRann behind the Assembly's room.
The 4 Quests are for 4 Sigils, from the 4 Keepers. You have to then do:
- 3 Minute Hodir Kill (I said that cache was rare)
- Thorim + Sif KIll (Use Your Illusion)
- Freya+3 (Knock, Knock, Knock on wood)
- Mimiron HM (Firefighter)
As for difficulty - it varies from guild to guild, but hodir/thorim are the easy ones, freya is hard, and mimron is well hard.
Once you have done these, i believe you get another item / quest to kill Alagon in an hour. Not really sure as we've still not got Mimiron/Firefighter, though hopefully he'll drop this week. It's by far my favourite encounter in the history of Warcraft. At the end of last week's attempts (about 10 hours total on him over 3 weeks) we can pretty much get to Phase4 regularly.
You can do this on consecutive weeks btw. Once you have the kill, you loot the sigil and you are done with that quest part. You only need 1 person who has doen all 4 top open Alagon's door and start the timer (or however it works) but of course, once you've done them once, you can do them more.
on a side note, i've come to the realisation that: I wish the Colosseum was interesting, but it's just not. Static Fights. No change of zone. No Trash. Just 3 piss easy boss fights (ohh, 4 now - wooo) and that retarded emblem system.