The Lightspring

For those having problems soloing this q, here's how i did it:
I chose light side points, which made me fight the sith and Diago Hixan. As fast as possible, nuke down Diago. He is stunned for a long time by our jedi friend, enough time to take him down to about 1-2k hp. When he wakes up, interrupt his heals. When he is down, you should be able to recouperate. Before trying to kill the sith, put your companion on passive. Then start nuking her. When she reaches about 30%, she will aggroe you. Have your companion taunt her off you, and just nuke her down.

I assume the same works for choosing the dark points option, thus fighting the jedi and Diago.

I totally agree with Diriana's approach... If, however, you happen to be struggling to put out enough dps to down Diago before he gets into healing and you find yourself out of interrupts to stop said healing, dropping combat (disappearing act for scoundrels) will cause him to attack whichever of the ladies you chose, and eventually aggro them.

Since both glowstick wielders are invincible (locked at like 9200 health until you engage them), you can afford to sit back, let them take the damage from both opponents, and use their added damage on Diago (once they're locked on) to easily nuke him down... then follow the above advice for the sith or jedi.

If, say, you're a healer or your companion goes down early, this may give you the second chance you need...

best suggestion is don't even attempt this if you're not about level 29. I tried both pathes 3X a piece at 27 and kept ending up being part of the floor.

If you choose light be prepared to deal with a lot of AoEs and try to interrupt her as much as possible.
If you choose dark it feels much easier but gets much harder later in the fight when you or your companion start getting weak.

Remember that the NPC other than the jedi is stunned for a good duration, this gives a chance to put some damage on him early on. Just leave a DoT on him at the least, interrupt heals or stun later on.

I don't know why all of a sudden they thought it was a good idea to put two elites on screen that are difficult to fight even with the stun early on, these NPCs will tear you a new one if you're underleveled.

This fight is (was) bugged for me. Managed to get the sith, no prob. Turned to Hixan and he and the Jedi are fighting. Send Corso in. He dies, and I can't res him because I'm in combat. I sit and watch the jedi and Hixan fight each other FOR TWENTY FRIKKING MINUTES, with neither one getting the upper hand, even with my help for the jedi. Interrupts don't work well with Hixan - he just heals and keeps on attacking the jedi. I don't think I can abandon, then re-accept the mission (or can I? Anybody know?). I can't quit normal, so I crash the game and re log-in. That resets Corso, but he dies again, all the same, and the jedi and Hixan ARE STILL FIGHTING, so I'm back to square one.

Left a ticket and crashed again inside the Lightspring, so maybe a dev can see what's going on, but this is just poor design in my book. There should be some way for a player without a companion to take him down. (growl)

UPDATE 3/3/12 Just logged in - they're still fighting away!

Corso doesn't have a harpoon attack anymore, starwolf. It was changed in a recent patch to a leaping stun attack. I would suggest turning that off so he stays out of AoE range for as long as possible.

I'm not certain if they nerfed this fight or not as of 1.2 but I was able to do it in substandard gear as a level 28 Gunslinger. The only items I had up to date were my chest and guns. I put Corso on passive and tried to pull Diago to the bridge without success. I burned him down to about 75% before I let Corso attack and interrupted as many of his heals as possible. He still defeated Corso shortly before I was able to defeat him. Moving on to Vaverone, I let Nariel tank her for a bit, though neither of them was putting much of a dent in the other. She's only a Strong class mob so I basically pulled out all the stops and burned her down using all my cds. One Pulse Detonator and two Distractions were enough to keep her from using her AoEs on me and I was able to defeat her fairly easily even without a Companion. Good luck to any future Smugglers getting this done.

If you side with the Sith (Dark Side Points), she gives you three options at the end:

1) Credits
2) Equipment
3) "[Flirt] You know what I want"

I picked Credits and didn't seem to get anything from it. Guessing very few people testing actually picked that option.

This took awhile for me to accomplish, none of the tactics used here seemed to work for me, mainly since Diego was already unstunned by the time I decided to come over here and see if anyone had some suggestions for beating this.

After several deaths I managed to find a strategy that worked for me, that allowed me to beat both the sith and Diego without dying(The same can't be said for Corso though) as a level 26 Gunslinger with gear obtained only through quest rewards, and no med packs or stims.

What I did was sit back and watch them duke it out for awhile, basically letting the Jedi generate a ton of aggro. Then I focused on the sith, since that seemed to be who the Jedi was wailing on. I sent Corso in first, and he fell right away. The Sith went down surprisingly quick with just me and the Jedi going to town on her. Once she was down, the Jedi turned her attention toward Diego, and I let her smack him about for awhile until she generated some aggro. Then I ran up to him, fairly close, and started attacking him. When he would cast his Med scan I would do one of the following: Confusion, Flash grenade, kick(or as a last resort discharge). He would still get a scan off on occasion during the brief period when all my disrupting attacks were on cool down, but after awhile we managed to kill him.

It was a pain in the neck, but quite doable.

As others found Dirana tactics useful so I did it almost 100% like that.

I started the fight with my companion on the bridge. Before I attacked either guy I had Corso harpoon Diago onto the bridge to keep us away from the Sith and her AOE. Then me and Corso just unloaded on Diago and by the time the Jedi stun wore off, I was able to stun him and finish him off without him even casting a single heal.

By seperating myself from the Sith on the bridge, I was able to recoup and rest ensuring both me and Corso were 100% health and Energy. Then I took harpoon off auto cast for Corso and just proceeded to unload with him onto the Sith, which conveniently died the same time that Corso did.

Also I made sure to stay away from Corso when he took agro off the Jedi to again, avoid the AOE.

On my Smuggler with Corso, the fight started with Corso and myself in the middle of it. I died a couple of times and it got to a deadlock when Diago, the Jedi and Sith were in point-blank range of each other.

After reading the other comments of strategy, the keys are to keep Diago at ranged, away from the J & S, and interupt his heals

I got into the middle of the fight to use Pulse Detonator to break the deadlock which worked, I then jumped straight onto Diago who had set himself up with his barrier away from the other 2. By some freak coincidence, I happened to stick right behind him so I could prop up my shield and be within melee range, but because the deadlock went for so long, he was fixated on the Jedi, and apart from Corso dieing on me, Diago never focused on me, so it took me about 5 minutes with staggered combos of flash grenades and dirty kicking. I didn't get to interupt all his heals, but I managed pump out enough DPS to gradually deplete his HP.

Once Diago was down, the Sith was a piece of cake. The problem is, as others have also pointed out, if you attack the Sith before Diago, though it looks you've come near her death, she'll bite back and kill you (might have been circumstantial for me as I was low on energy to effectively DPS her at the time, considering Corso was already down).