Darth Venamis vs. Exar Kun

Battle takes place on neutral ground

Exar's more powerful and more skilled. Venamis's speed and versatility makes it a fight but it won't save him.

Where do you place Kun on a speed tier? Venamis is exceptionally fast

Enough to react to Venamis. The speed edge is pretty apparent at the start of the duel, but wanes throughout the fight. Eventually it'll be next to useless, but one of them will die before that happens. I don't know - Kun's more talented with a blade and more powerful in the Force, which are the primary edges here. This is assuming Kun has his usual gear, of course.

Didn't Luke Skywalker claim that Exar Kun was the most powerful Dark Side energy he felt sense Sidious, at lowest he should be as strong as Darth Vader. Also he nearly beat Luke Skywalker while in posession of Kyp Durron

That was Force-only, not a duel. Obviously Venamis can't keep up with Kun's Force power, but in a duel, it's closer.

Venamis is skilled and fast enough to make Kun really work for it but not much more.

Kun crushes.

If Plagueis didn't crush Venamis, then Kun won't either.

Plagueis' failures are his fault, not Kun's.

Kun will share them.