Based off of the Hammerknell raid, the goal of this match is to reach 1,000 points to win. In order to do that, you need to capture the runes placed around the map, and hold onto them. Think of it as a CTF, or Capture The Flag match -- with the exception of there being six flags instead of just one.
From the start, you will find 2 runes on both sides at ground level and the basement level. Towards mid game, another set of runes will be added at the bridges for a total of 6 in game. So when the game starts, you should split up the group so that you can cover both sides. Holding on to these runes will give you the points you need in order to reach 1,000 and win.
The second way of getting points in the game is through killing enemy players, but in comparison to holding the runes the points are minimal. So if you do have the runes, you should group up with your teammates for support and healing.
If you are trying to kill the enemy players who are holding the flag, it would be useful to use abilities that cut down on steal healing. These things, coupled in with the damage dealt from holding the runes themselves, will help out with stealing the objectives.
People who can grab and hold on to the runes should switch to tanks for their high health and armor. Otherwise, it is better to have a few key DPS and healers. Single target damage is better than area of effect, as well as abilities that cut down healing as mentioned earlier.