To access Sher Kar Cave you first need an Omniglobular Syringe created by combining.
A Faintly Glowing Slave Collar, A Faintly Glowing Booster, and A Faintly Glowing Patch of Hair using a Chu Gon Dar Cube to create a Warmly Glowing Dispersal Unit.
Then Combine A Warmly Glowing Dispersal Unit, A Warmly Glowing Poison Gland, and A Warmly Glowing Reactive Antidote using a Chu Gon Dar Cube to create an Omniglobular Syringe.

This means you should have completed the prerequisite Final Battle of HK-47 as he drops the Warmly Glowing Reactive Antidote.

Next you need to travel up Tulrus Canyon to the Cave of Sher Kar.
/way mustafar 805 86 1286 Sher Kar Cave;

Click the big boulder and you will enter the cave, you cannot enter without the Omniglobular Syringe.

Inside you will find Sher Kar and 4 mini bosses.

Kill a karling, 2 praetorian, and a symbiot before the final battle with Sher Kar.

Personally I like battling Sher Kar as it is a good gauge for how much DPS I deal with a character build.

This particular build is dealing just over 8k DPS after stacking a few debuffs on Sher Kar.

Sher Kar provides one of the fastest ways to get credits. Also drops 2 Handed Old Republic Lightsaber Schematic.

100,000 Credits and 1 Mustafar Token of Heroism.