As a marauder you will be pitted up against Ferocity Form VII, Juyo when fighting the 3rd and final (live) jedi master. This form makes attacks MUCH stronger with a lightsaber. However, it has a drastic weakness to force powers. You are fighting an uphill battle lightsaber on lightsaber. Make your defense as strong as possible, and unload "with great vengeance and furious anger" on your adversary with force powers. Higher charisma ensures you'll CRUSH them.
Use form either form III Soresu or form IV Ataru to increase your defense for this situation. Form II Makashi might work but it does nothing to bolster your defense.
This form actually makes Vrook a ***lot*** easier to kill. It won't do much to bolster Kavar either--he's mediocre anyways no matter what.
The only one I'd be a bit cautious about fighting against while he uses the Juyo form is Master Zez Kai Ell. Zez Kai Ell uses critical strike and that is what attack form VII Juyo improves most--it increases chance to stun and it does something like 2.5 times the damage. In fact his attack power will even exceed that of Vrook and Darth Sion--he might even be a 1 or 2 hit kill against you. Force wave, keep him away and stunned. DO NOT let him near you. When you learn the form halfway through the fight, THEN you can go lightsaber on lightsaber against him and "show him how it's done" as you finish him off with malice--or continue assaulting him with force powers.
Note: When YOU use it, make sure you use the power Force Shield to cover this vs. dark jedi. Best if used briefly unless you are POSITIVE you'll wipe out all your enemies, because over long periods it also leaves your physical defense down against melee attacks. The theory behind this form is that it makes your lightsaber strikes to devastating that your enemies will not likely be standing after one or two passes.
If you'd like I can give you an analysis of the Jedi Masters and how to fight them.
Electra: "Let's go...together"
Vincent: "When I was with you, it was the only time I felt real"