I like KOTOR2 a lot but I have always hated the two villain's abilities. What happened here is... if an author is not competent to write a cool character then he tries to make him cool by giving him some ridiculously powerful abilities. The end result is usually something stupid.

Since I like KOTOR2 I compensate this stupidity by just reinterpreting it. For example, in case of Nihilus I interpret his "devouring of worlds" not as force ability but rather just a sort of a fetish, i.e. he force-feeds on the destruction that he is causing with his fleet and army. So in that case it makes the entire thing acceptable and his defeat in 1on1 combat feasible.