Well, technically Garen Malek and all the DLCs are considered non-canon by Disney, so all this is merely conjecture. He possessed a very strong connection to the Force, but he was even more skilled with a lightsaber. In a fight against a young Jedi Knight Anakin Skywalker, he would likely lose. His connection to the force, while exceptionally strong, was nothing compared to Anakin's, and his offensive Lightsaber style would be countered well by Anakin's own. They're both agile fighters, who frequently use telekinesis and force augmented abilities, but frankly, Anakin uses them better. In a fight against Darth Vader, however, he would win almost every time. Vader's midi-chlorian count was drastically decreased when he lost all of his limbs, and his suit greatly hampers his dexterity. Vader's lightsaber form now focuses on raw strength, which would easily be countered by Starkiller's agile form, and Starkiller's force ability now surpassed Vader's. Hopefully that gives you a good estimate of his power. I haven't played the DLCs, but I can still give a good estimation of his power.

Grevious was able to take down the Jedi he did through a combination of multiple factors. 1) Underhanded tactics. He had no issue with things like having a MagnaGuard stab a foe in the back or a sniper shoot them down during a fight. 2) Superior stamina/No fatigue. He could theoretically fight for as long as it took, while his organic opponents would wear down and run out of energy. 3) Sheer, raw power.