The Jedi Master Shaak Ti is the boss on the first incarnation of the planet Felucia. She is tough, well-versed in the force and has a giant Sarlac on her side in addition to a cadre of Felucian soldiers. Beating her won't be easy.
Keep to the middle. You probably noticed the bubble-like nodes around Felucia that explode when you walk over them. They are present when you fight Shaak Ti as well. These can be a double standard for you. On the one hand, forcing Shaak Ti into them can take a huge chunk off her health bar. On the other hand, they can take a huge chunk off your health bar. Using them is a risk, but can pay off if your Lightsaber combos are good.
Use Force Repulse. This works wonders for clearing the Felucians from you. This will be very important, as the last thing you want is to fight a score of Felucians along with Shaak Ti. She will summon Felucians to fight you at several points during the fight. Get rid of them with Force Repulse or Grip (throw them over the far edge.) Always kill the Felucians before you go back to Shaak Ti.
Learn to hit the button. Starting with Kasdan Paratus, many bosses (including Shaak Ti) can resist your force powers. To overpower them, you will need to press the displayed button when the cursor enters the small highlighted zone of the circle. This is difficult and you will need to get it right several times to overpower your opponent. The trick is usually to hit the button a little before the cursor reaches the highlight zone.
Use Sith Saber Slam. Shaak Ti is adversely affected by electrical attacks. If you are using the Firkrann crystal, this will help keep her stunned for a short time after you unleash a Saber Slam. These few seconds are critical for using force powers on her.
Dodge the Sarlac tentacles. These massive limbs can crush you for heavy damage late in the fight. Avoid them by watching them for a writhing movement. Double-jump and dash to one side when you see it coming. Keep doing this until Shaak Ti comes back down.
Avoid using straight Force Lightning if you can't get the timing right on overpowering. This will save you a headache. Use Force Push or Force Repulse instead.