A lot to cover here.. Seeing how a lot of things can kill you and cost your team the kill.
The Ambassador mainly uses Ranged attacks with an occasional high hit magic attack. Ranged attack:
Special Attacks and how to avoid getting killed.
The first attack we encounter is a fire patch (just like the first boss in ED1); Run close to The Ambassador, wait for the timer to run out, the attack to drop, and then run out. This attack, unlike it's ed1 counter part, spreads out (meaning you could enlarge it), do wait a few seconds for it to actually drop.
Unstable Black Hole:
This one is quite simple, all you've got to do is stun it. If you fail to do so and are to close to it (it has a HUGE blast radius) you will take up to 8k+ damage.
Sinister Fragments AND VORTEX SPIN/Big Bang Attack:
This one is kind of tricky, you have to take out Sinister Fragments whilst running counter clockwise, trying to avoid a spinning beam!
If you've successfully taken out every fragment, nothing will happen afterwards.. If you're not so lucky, The Ambassador will use his Big Bang Attack (fake name), he'll do so depending on how many fragments he absorbed. If he managed to absorb 3 fragments he'll do this attack 3 times, within a few seconds, hitting 8100 each attack.
This can be blocked by barricading (requries a shield), using Debilitate (halves the damage) or using reflect (halves the damage)
Timer looks like this:
The last phase of the fight is where it gets harder. You'll have to pay attention to the chat to prevent being instant killed.. and also keep dpsing the boss whilst he's being healed!
The main thing that can kill you here is not paying attention is getting smack by high hit magic attacks (can be blocked by protect/deflect magic + devotion)
Watch the chat at all times and switch to the correct prayers until the attack is over (it's 3-4 hits) ((See screenshot, black balls are the special attack))
Word has it that standing between the hands blocks the healing effect on boss, I haven't fully tested this, however this shouldn't be an issue if your team's dps is high enough!
That's about it.... Have one more post-kill screenshot.