The Whitefall Steppes is a Capture the Flag game, with each side aiming at grabbing 3 flags. Each of the flags, or sourcestones, spawn at regular intervals depending on if the flag was turned in or not. The first way you can win at this match is if you have 3 flags turned in, which is an automatic victory. The second way is to have the most sourcestones turned in by the time the timer ends.
The sourcestone spawns in the center of the map, and has two main behaviors behind it too. The first is that if it is turned in, the next one will spawn after 30 seconds. The second is that if it isn't turned in the next sourcestone will not spawn.
If you hold the sourcestone, you will have 2 debuffs placed on you, one that cuts down the healing your receive, and another that makes you run a bit slower. So single target heals help here, as well as movement speed buffs. If you use any type of "cleanse debuff", it temporarily removes the movement speed debuff too.
Strategy here splits up for situations. The first situation is control of the center. If you have the sourcestone but do not control the center of the map, don't turn it in. If you do, then the next few spawns will go to the enemy team and they will win.
If you do have the sourcestone and you have control of mid -- but both teams are on the low side of 7 to 5 people, also don't turn it in. Because the funny thing about Rift is that when people are added from que to a match, the numbers can change the tides pretty quickly. So unless you are confident of your team, it is better to play it safe instead.
If you have control of mid and don't have the sourcestone, then you can pick up the remaining spawns and send thieves to steal the sourcestone from their base.
If you have the sourcestone, but you do not have control of the map, you should move your entire team close to your base, but do not stay within line of sight of the goal. The reason is because it is possible for the enemy team to pull you with tank abilities and force you to turn in the flag. If this happens, then they have 30 seconds till the next spawn comes and they can steal the sourcestone from you for two points.
And finally, if you don't have control of the map, the timer is counting down to 0, and the enemy team has 1 or 2 sourcestones turned in there is one thing you can do to force a tie. A point is scored if the sourcestone is placed at your or the enemy base. But if the flag is held by any team, it becomes temporarily neutral. So it is possible to steal both flags at the last moment and force the tie, which gives better rewards than a loss.
For this match, support builds trumps all. Support isn't usually played by players, who pretty much want to just do damage. But if two teams are evenly matched, the damage boosts from supports will make the difference.
The second most important thing is that it is better to have single target DPS than AOE (area of effect abilities). But this depends on if you have control of the middle or not. If you don't, area of affect abilities like DOTS (damage over time) can help cancel the cast time when picking up the sourcestone.
Also, AOE damage can help hit a larger range of targets. For instance, if you have a hit range of 30 meters, and if an AOE has a radius of 20 meters, that means that if you throw down an ability at max range you are also hitting targets 40 meters away. But since that damage is weaker individually, it's better to go single target DPS.