Instructions
Make sure the area is clear. There are several Scrap Drones and Junk Guardians around the first Junk Titan. The last thing you want is to fight the Titan with them still around. Make sure they are all destroyed before you fight the Titan. Note that shocking the big engine is what triggers the Junk Titan. Do not shock the engine until the area is silent.
Throw things at them. There will be a bunch of power cells and other objects near the first Junk Titan you fight. Grip them and throw them at the Titan for good damage. You must be quick! The Junk Titan has a ranged attack that hits for a ton of damage and can slow you down. Be aware that you will not have the luxury of throwing things at later Junk Titans. After the first Titan, try defeating the second one without throwing things at it.
Empty your Force Lightning into them. Double-jump up, then hit them with a full bar of Force Lightning. Start slashing away with your lightsaber on the way down. This is the best way to whittle them down once you run out of things to throw.
Keep jumping. Like the AT-STs (and their more powerful cousins), the Junk Titans can knock you down with a stomp. This is easy to avoid by staying in the air. Keep jumping and using aerial lightsaber combos on the Junk Titan.
Equip damage-boosting lightsaber power crystals. The extra electrical damage crystal (Firkrann, found on Raxxus Prime) is the best choice. Any extra electrical damage you can deal to the Junk Titan will be tremendously beneficial.