There are two easy challenges to snag this trophy on: the Kamino Grounds challenge and the Terror Trial challenge.
This is the challenge where storm troopers appear from two doors at opposite ends of the area, and run to two other doors at each corner of one wall. Fairly straight forward objective: stop the troopers from getting through the doors they are running towards. The challenge will end once 20 troopers get to their destination, and the platinum time for this challenge is 4 minutes.
I found that fully upgrading force lightning is your best bet here. Standing in front of the doorway that the troopers are running towards and zapping them will kill most of them, leaving one or two for you to cut up with your lightsaber. Then you dash your ass to the other doorway and repeat. Using the crystals that reduce the consumption of force power with abilities, and the one that recharges force powers upon striking enemies will help keep your force power bar up.
I got to 4 mins only letting 4 troopers get through their respective doorways with this method. TERROR TRIAL:
Another fairly straight forward challenge - you need to defeat 3 or 4 waves of spider droids and terror troopers (the ones that disappear and reappear all over the place). The platinum time for this challenge is 1 minute 15 seconds.
I used the same lightsaber crystals as the Kamino grounds, but instead of force lightning I used force repulse (for the spiders) and mind trick (for the troopers). Once a trooper is under your control it is easy to cut them up
Also note that impaling a terror trooper is an instakill (grip + saber throw). If you are down to the last terror trooper and just need him dead that is the way to go.
I had this done at 1 minute 12 seconds, and to be honest I was only gunning for the gold medal (1 min 30 seconds) so this was a nice surprise.
If I missed anything, or if anyone else has tips on getting a plat on another challenge, let me know in the comments and I will update this solution.