This is a page that describes how to run the Abbadon's Gate Mission for Masters.
Theo adapted this from GuildWiki...
The concept is to use the monk skill
and/or the Ritualist skill
to have Abbadon Kill himself. The Reversal of Damage is preferred as it stops damage to you and steals health from Abbadon.
You need a party of 4 heroes that are strong enough to kill two groups of two monoliths. All four heroes need to have Monk or Rit secondaries with either of the two skills mentioned above. What you will do is enter the Mission, run to Abaddon's chamber and flag your heroes in the center far enough back not to aggro the two monoliths near Abaddon. Then pull the two monoliths on the right. You can ignore the torment claw. As soon as the monoliths are dead pull the two monoliths on the left. Then stay there and just cast the RoD skills on you and your heroes over and over. You will get the timing very quickly. Wait about 4 seconds after Abbadon has cast his Words of Madness skill and then cast it on yourself and click each of your heroes to cast it. Repeat over and over and watch Abaddon die pretty quickly. Just make sure you do not get in aggro range of the last pair of monoliths near Abbadon.
It is advisable to have LB up and to bring Lightbringer Gaze. Neither one helps with the Monoliths or Abaddon but they will be a big help in case a Torment Claw pops up on you. Normally running into the middle and pulling you will not have to deal with a Torment Claw, but every now and then one will pop up just to make life a little more interesting.
I used a group of Dark Grendl WaMo, Goren WaMo, Melonni DeMo and Raza Rit. I also added Mhenlo and Kihm as henchies. I completed the mission in 13:52, Masters is < 20. The only thing special about the Builds is that they have to do enough damage to kill the four monoliths and then have enough points in Smiting Prayers or Restoration Magic to have ROD or VW do 50 points of damage or more.
Just did it in Hard Mode and got Masters. I had to make an adjustment and bring two more henchies along to help with killing the Monoliths. They are tougher in Hard Mode. I added Cynn and Gehraz. I was afraid Devonna would run off in spite of the flag and aggro the last two monoliths. I would also dump any rebirths and bring Rez Sigs for this. Razah died during this and we couldn't rez him with rebirth, it kept getting interrupted.
As another note Theo did this mission with just a party of 4 and still got Masters.