Warframe

To earn this achievement you have to finish five missions for a clan that is currently stuck in a Dark Sector Conflict. A few notes about this achievement:

1. Your clan doesn't have to be in the Conflict, you can just pick a side and fight for them. Technically you don't even have to participate at all but I found that if I didn't kill at least a few NPC's then it didn't count for me.

2. At the moment, likely due to a bug as the achievements in Warframe are indeed very buggy, you don't have to choose the clan that wins. Any five wins will get this achievement to pop for you. You don't even have to choose the same side five times.

3. Because this achievement is buggy, you might end up with it not counting properly when you participate in one. If this happens to you be sure to kill at least one PC as well as a few of the NPC's. If this doesn't help try to hard shutdown your Xbox (hold the power button until the Xbox shuts down) and then try again but at a different Dark Sector Conflict on the off-chance that the fight itself is glitched.

To explain the game mode itself and provide some tips for those who actually wish to participate in the fighting:

Dark Sector Conflict
It is designed in a similar fashion to a MOBA - the players are battling each other and only they can take objectives, the NPC's are basically there to get in the way or help as well as providing "experience" for you. Your goal is based on which side you are part of.

Attacker: You must reach two terminals on the outside of the Solar Rail and hack them. Afterwards you must guard them for about 30 seconds until they stop flashing. Once both of those are hacked you must repeat this again in an interior area. Lastly you'll get into an innermost chamber where you must hack two consoles (no defending them this time) and then attack the orb that powers the Solar Rail. Damaging it enough leads to a win, destroying it only really helps the attacking clan / alliance more.

As an attacker you lose if all of your lives run out. The game says you have 20 but this doesn't seem to be accurate. In a few missions I've died a number of times and haven't seen it dwindle. It may only be that it only counts if you're finished off while downed and shooting but I'm unsure. It might just be glitchy.

Defender: You'll simply need to hunker down and defend these terminals. If some grunt manages to trigger a terminal then you'll want to quickly do so yourself so that you disable it. If they push their way into the innermost area you'll simply need to keep killing them as fast as you can before they damage the rail too much.

As a defender you only lose if the attackers manage to deal too much damage to the power orb. All you really need to do is to hold them off.

A Warning About Mods
When you enter a Conflict all of your mods are stripped from you except for a rank 0 version of your basic ability. You re-earn these by earning "experience" which is in turn gained by killing NPC's, gained in greater number by killing PC's and you can gain a bit by participating in hacking objectives or attacking the power orb as an attacker. These mods will reappear in bursts as you fight enough so it's in your best interest to kill the NPC's to earn some basic ones back and then go after the PC's. If you can pick off PC's quickly then it's all the better.

Warframe Choices
When choosing a Warframe it's a kind of tricky one - many Warframe abilities are made much less effective in this mode. Valkyr's Hysteria will only last about 1/10^th the time so while you can murder the heck out of people is nice, it's a minor boost. Many other abilities have this same effect. As such you'll want to choose between one of three Warframes that still retain a great amount of utility:

Rhino: Iron Skin is still as useful as it ever was so you'll want to go for a Rhino with specs to take advantage of it. So go for Power Strength and Power Efficiency, increasing the HP of the skin and reducing the cost. This extra boost will allow you to get in close to stun lock people while preventing you from being stun locked as easily.

Mag: Yes, Mag of all Warframes. Pull works just fine on enemy Warframes unless they're using Iron Skin, Hysteria or some other knockdown resistant ability. As such a Mag specc'ed to take advantage of Power Range and Power Efficiency so you can do it from a greater distance and use it quite often.

Vauban: Both the Tesla grenades and Vortex are incredibly helpful against the enemy players. Sticking grenades to your target, like the consoles, will help to keep attackers off or ward off defenders who wish to stop you. Vortex is just amazing. Spec both for range for optimum results.

Weapon Choices
Considering you're dealing with PC's don't expect any decency or fair fighting. As such get ready to be as cheesy and unfair as they're going to be. The best thing you can do is pick up Dual Zoren or Dual Ichor, specc'ed for high critical hits chance and attack speed. Basic hits will stagger the enemies perpetually since you attack so fast and the critical hits will help you kill people faster. Guns work as well but they're not helpful when you first get them. However if you pick up something strong like a Dread and can aim it properly, a high critical Dread will murder the crap out of people.

EDIT: 10/24/2014 - As an addendum to the achievement not unlocking: I've been scouring a few forums for information on this topic and it seems to be a personal issue some people are having. As in the achievement can still unlock, someone in my clan just did it while idling in the spawn room while we fought, but for some people it absolutely refuses to progress. I'm not sure what's causing it. The only solution I can offer in this regard is to message DE on Twitter or visit their forums to ask about it. This might make it so that they issue a hotfix for the problem in the near future or at least address it in a patch.

EDIT: 10/26/2014 - Hunter XBA has pointed out the following details that allowed him to get the achievement recently: He put together a squad, meaning he was host, and started a Dark Sector as the attackers. He saw himself get dropped off by his ship and his squad proceeded to win the round. This is what got him the progress that allowed him to get the achievement. However another user has stated that he did this and didn't get any achievement progress so it may be that this works for some but not others. If you're stuck for progress it's worth a shot.

After waiting for months to get this final achievement, the latest update on August 5th finally updated this!
As of the latest update, the achievement description has been changed from "Complete 5 missions for a Clan that then wins a contested Dark Sector." TO "Complete 5 Dark Sector Missions."

Technically, this can be completed in less than 20 minutes if you quit each mission as soon as possible and you can steamroll lower level enemies!

Thanks to the commenters(Geoffistopheles, x CreaM v) on this solution that there were good missions on Earth, I finally got back to updating it. From what I just tested, I was able to complete a Dark Sector Excavation on Earth in about 4 minutes, from the start of the first loading screen, to mission completion. Also, I had to remember the controls so I was a bit clumsy too!

This appears to be by far the fastest method:
Mission: Dark Sector Excavation
Faction: Grineer
Level: 5-15
Sector: Tikal

If your gear isn't up to it for the Excavation, then this mission is a good alternative as you don't have to defend a specific point but can move around a lot more! Also, you only need to go 5 minutes before you can extract so quite fast also.
Mission: Dark Sector Defense
Faction: Infestation
Level: 5-15
Sector: Coba

If you're interested, here's a detailed listing of my warframe and mods:

As of Update 16.3, this achievement is unobtainable!

Since Conclave 2.0 (PvP), there is a "temporary" armistice on Dark Sectors nodes and no conflict until the new Conclave system is approved, tweaked and spread everywhere.

Edit : U16.5.9 out. Dark Sector Conflicts still inactive
Edit : U16.7.2 : Still inactive (Thanks to "Chris Scarce" for pointing that out)

Edit : U16.11.4 : OBTAINABLE AGAIN!!
"- The Dark Sector achievement has been changed to fix the achievement being unobtainable due to the Armistice. It is now obtainable by completing any 5 missions on an uncontested Dark Sector node. This change does not affect players who already had progress in the achievement."
(Also pointed out by "LSK ArchAbyss" ;) )

What misfit119 said is spot on. I had a slightly different experience though. When I tried defending on a dark sector, it wouldn't let me. It kept saying that it couldn't find other players, so I only attacked.

I won five times I attacked, and the achievement didn't pop, but my progress bar went up after every win. After the dark sector conflict was over, the clan that was defending was the victor, but my progress bar stayed up.

The next time that clan had a conflict on the same planet, I fought again for the attacking clan again, it was a different clan that was contesting the sector than before. I fought and won only one battle for this clan and the achievement popped after the win.