This. Seriously.
Right now, my headcanon is saying our characters just need to fully recover from residing in that carbonite then we'll wipe the floor with Arcann and Vaylin. We've faced Soa, Kephess, the Dread Masters, and Revan himself. We've survived and fried bigger fish basically. Even if Arcann has more raw power than the Force using PCs, we have far more experience than him.
Zakuul overall is just laughable. I found it amusing how despite their apparent overwhelming numbers, according to Lana, they relied on ambush tactics during the war. Concerning the Knights of Zakuul, the only advantage I've seen thus far that they have over the Jedi and Sith other than the apparent better Force usage (Which we really haven't seen) is choice of weaponry.
In the end, I don't think Zakuul is that well-written if at all, considering how they just pop out of nowhere to crush both the Empire and the Republic just like that with no hints of their existence at all beforehand.