The simple fact is that you need to learn from your mistakes. You need to learn to utilize interrupts to stop enemy inductions, you need to learn to upgrade your gear as best you're able (all the time, it's a treadmill), you need to learn to purchase the best possible consumables... and you need to learn how to put your skills together in a significantly beneficial rotation.
For those looking for some inspiration, this video helps (was mentioned before):
Watch this video closely during the fight (last three minutes, roughly).
First thing to notice is to watch Valis' life bar. Note when the Blue bar begins to progress below it: that's him using a channeled skill that you can interrupt.
Second thing you'll notice is that Force Stasis was never used.
There are several mistakes that the player actually makes, yet still succeeds at level 24:
No Stim, Endurance or otherwise, is applied during the entire time. This is certainly simple to change -- stab yourself with a needle of any description and you're already money ahead.
The entire battle, you'll notice, lasts only 60 seconds. Saber Ward should have been popped with the initial aggro, followed immediately by Call on the Force which would have set up a Heal over Time for the entire duration of the fight on both the player and Kira, not to mention reset Saber Ward instantly for another use.
When Kira had reached roughly 30% of her health, the player should have Taunted Valis back onto the player character. Yes, you'll be taking damage instead of Kira, but Kira constitutes a very large portion of your total DPS output as a Team -- Keeping her up allows you to put more damage onto Valis. This is a DPS race, more or less... and keeping Kira alive and swinging is important.
The choice of stance (Shii-cho) is questionable. At level 24 a Guardian would have access to Soresu since forever, and even Shien if they were 11 points into the Vigilance tree. A Sentinel would have the option of either Juyo or possibly even Ataru.
The player seems to insist on doing their AOE Sweep when a more damaging single-target skill is available and off cooldown. One could chalk this up to the stress of the situation. However, if you're playing this particular battle, please do remember that he's a single target.
They did, however, do a wonderful job of using Interrupts (both from Force Kick and from Force Leap) and used a top-of-the-line healing consumable.
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TLDR; Go back and read.
Further, when you fail, stop and think about what ELSE you could try, rather than continuing to run full-bore into a brick wall and wonder why it continues to fail.
Hopefully this helps and somehow avoids the bruised egos of some rather tender-skinned individuals that have posted earlier.