Boss one in Ulduar one on shotted.
It was rather confusing and messy, I got to drive a Seige engine so when I was targeted by the Leviathon I got my little bolts out of the way and ran for the walls.
My one complaint is when you are the target, if another seige engine gets in your path and you try and use your haste button it gets wasted as you only run into the back of them. It can get quite confusing, with all the smoke, and even though I had my spell detail down It was hard to see stuff.
We did however unintenionaly get the achievement.
We did wipe on trash though on the way to the next boss – their little flame funnels type things hurt, and they chase you.
And then there is that the server is now going down for 2 hours. /cry So the 1st boss was it for our raiding.
Provided by 
Pugnacious Priest's Warcraft Blog 
Congrats! Hopefully people will post plenty of tips etc. Was it the server first?
Our faction had control of Vault and so we attempted the new boss (it was late – Emalon?). We wiped a couple of times and, unfortunately, we got the message that WG would be starting shortly and that we would be ported out when going in for our 3rd try. Oh well.
no, not a server first. Apparently someone had already killed the 2nd boss, and at least 4 guilds were in there at 11am our server time. Badluck about WG 😦
Grats on your first Flame Leviathan kill! My guild on Moonguard got to try it on 10 man last night. Managed to down it after 3 attempts. So far the general consensus is that it is an actual “fun” boss fight. So much to do for each person, and it’s a bit new.
Good luck on the rest of the instance.
Don’t you just love unexpected acheivements, it’s like getting a present a week after your birthday!
Belated Gratz on the kill.