You have an answer in Jedi Academy: Exar's spirit kicked Luke's ass in matter of seconds. Later he was destroyed by Luke's student with aid of Master Vodo's spirit.
But it took 14 Jedi to destroy his spirit and thousands to destroy him when he was in full power. While it took 2 Jedi to finish Palpi
And don't forget that Exar as a living being, not a spirit, was far more powerful than his ghost. And yet this ghost threw Luke into coma.
And please don't tell that this is lame EU. Anderson sent his question about Exar Kun to Lucas and Lucas said "make him a Dark Lord" (he was supposed to be Dark Jedi in the beginnning) and also made a list of abilities that Exar and ancient Sith were intended to have.
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When did that happen in Jedi Academy?? If you mean the game that is..