For the Black Garden: Domination, you need to capture any of the three objectives and hold them for the longest time possible. In order to win, you need to reach 500 points. Points are awarded for holding an objective for every tick. Killing enemy players will also give you on average around 2 points per kill.
On either side of the map there are two buffs. Each buff will boost your damage, so if you are under control of the map (or if you aren't), rushing to grab the red orbs can change the tide. From the start of the match, the best thing to do is to split the groups into two and grab both of the outskirt objectives.
If you are on the Guardian side or Defiant, when reaching for your objective you will need to keep one person to grab the point as quickly as possible, and the rest of the sub-team to be prepared to intercept incoming players. Afterwards, always make sure you have one or two players holding the post.
Now if all goes well, you would have two main points and it would become a war of attrition...except that the enemy team has to capture another point in order to win, so they will split up. When they split, that's when you should grab mid, force it to convert, and go back to defend your point.
3 healers, 1 support, 6 damage dealers. Area of Effect attacks help stop the enemy from stealing the objective, but single target attacks are better for taking down targets one by one.