Let's keep this short, sweet and to the point.
Desann's Force Abilities:
Desann's tutelage under Luke speaks of the young Chistori's Force Strength, as Luke often sought to teach his strongest students on a more personal level than how he trained others. Example's being Gantoris, Kyp, Kyle, Jacen and Jaina. It's not unfair to say that there's an obvious pattern in Luke's choice of students, and their respective abilities. While that may not be proof enough of Desann's Force Strength, we have sources saying that not only did his Reborn Students draw from the Valley, but both Desann and Hethrir also drew from it, effectively quadrupling their innate Force Abilities.
Desann and Hethrir both used the Valley of the Jedi to increase their power.
Hethrir increased his powers fourfold by absorbing energy from Jedi and Sith spirits. After gaining that power, the two Jedi were able to imbue others with the power of the Force.
—Star Wars: Threats of the Galaxy
Please keep in mind, that prior to finding the Valley, Desann was capable of this:
Furthermore, merely drinking from the Valley gave the Reborn extremely powerful abilities.
"Reborn warriors are basically jacked up versions of the basic Reborn Adept, and use more Force Powers than their less experienced brothers."
—Star Wars: Jedi Academy: Prima's Official Strategy Guide.
-----
"Master Reborn are the most advanced of all Reborn, and use a variety of Lightsabers with all available styles. They have ALL available Force Powers."
—Star Wars: Jedi Academy: Prima's Official Strategy Guide
This is further confirmed by the Jedi Vs Sith: Essential Guide to the Force.
Allying with Imperial admiral Galak Fyyar, Desann began experimenting with Sith alchemy and artusian crystals to artificially imbue human Empire Youths--who had previously displayed little or no talent in the Force--with dark side powers. Brought to Ruusan, the Empire Youths found their innate powers had quadrupled.—The Essential Guide to the Force
So his Strength in the Force, which allows him to Augment his Speed and draw on the Force to create blasts of Power, has grown substantially(You're welcome DC 🙂 )
Given that Shaak Ti lacks any vital TK feats that contain the same raw level of power, She is significantly out-gunned in this regard by Desann.
--------------------------
In regards to Shaak Ti's dueling ability as of TFU, her skill is not within doubt. As I'm sure Desann's skill is like wise acknowledged by you, after all.
However, Desann was defeated by a man who had a plethora of knowledge of Lightsaber combat given to him by the spirits of Ruusan. Not to mention this man defeated countless of Reborn warriors all personally trained by Desann in groups, and was given a position by Luke that acknowledged Katarn's mastery and knowledge of the Lightsaber, which evidently surpassed that of Kam Solusar who was a capable warrior in his own right.
But, I wouldn't be doing Desann justice if I didn't mention his feat of dueling Luke Skywalker twice. So, I imagine I've left most people scratching their heads whenever I say they fought twice. Well, thanks to some deep digging, I found this neat little quote:
“I managed to scatter Desann, and his Reborn troops, and Rogue Squadron chased away their ships. For now the Valley’s effectively sealed off.”
—Star Wars: Jedi Knight: Jedi Outcast(Luke Skywalker)
If Luke had merely scattered The Reborn and Imperial remnant, I would imagine that saying that instead of mentioning Desann by name would have been more appropriate, not to mention it would obviously explain how he got to the Doomgiver in the First Place. So Luke's fought Desann twice, admittedly on ambiguous terms in both occasions, but they fought none-the-less.(Again, you're welcome DC 🙂 )
Desann's fight with Luke, while randomized, is still a show of skill and Force prowess. Given how Luke, even at this stage, isn't a mook. What's more, Luke considers Desann a major threat, given how he attempts to warn off Katarn.
"If you (Kyle Katarn) run into Desann, don't try to take him on alone. Now that he's been to the Valley of the Jedi he'll be more powerful than ever."
—Star Wars: Jedi Knight: Jedi Outcast
This isn't the same scenario where Luke is forced to battle Brakiss or baited into a Trap. Luke actively chose to go after Desann and warn off Katarn, who has regained his connection to the Force. Evidently thinking Kyle needed assistance against Desann.
Another example of this being:
Kyle:“Does this mean we have a new Emperor on our hands?”
Luke: “It may be worse than that.”
—Star Wars: Jedi Knight; Jedi Outcast
Rarely, if ever, in the narrative of the Star Wars Expanded Universe, has Luke considered people like Brakiss, Tamith Kal, and Nyax(?) even remotely worse when compared to the Emperor. Among the people Luke thought were near or close to that same level, is the unseen threat of Exar Kun, Lord Taalon, Abeloth and perhaps even Krayt(?) So while you're content to dismiss the quote as hyperbole, in the end the quote does speak of just how powerful Valley of the Jedi Amp! Desann is, and further adds to the fact that Luke personally fought Desann on more than one occasion. While I wouldn't dare to ever suggest Desann is close to Sidious, it is a statement rarely uttered by Luke unless he considers it a true threat. And seeing as the Valley before hand was capable of giving the likes of Jerec nigh-omnipotence, It does stand to reason to ask if it's fair to assume that the quote is hyperbole, but I digress.
Personally, as someone who has defended Shaak against Luminara in a debate with Joker. I don't see how Shaak Ti would be able to best Desann, but that is why I now leave the floor, to you.
EDIT: KMC's acting up on my end, the gif won't show so here's what I wanted to show
Fated has the most peculiar way of phrasing things in debates that I would best describe as a pseudodramatic pseudoformal blend.
That being said, I was more impressed by his post than I expected to be, so props to you Fated 👆
This is an interesting debate in the sense that both opponents are intimately familiar with their opponent's character. Fated has debated Shaak vs Luminara sabers only with Joker, and seen Shaak lose to Kas'im in a debate I took part in. Likewise Wollf is familiar with Desann from when he was going through sources to make a Kyle Katarn RT.
I look forward to Wollf's response.