Decades of Jedi killing experience and the ability to fight on par with the Exile. Combine that with the fact it takes a very hard beating for him to go down and simply put Barriss Offee is no Exile.
Also can we not pretend Offee was fighting on par with Anakin? Half the time she ran away and the rest of the time he was beating her down.
Except for when she kicked him off a ledge and almost murdered him there, there's that part. Or the part when she almost beheads him at the end, that maybe kind of should be mentioned.
Yeah, but idk much about Sion so whatever, lol.
Edit: Experience is so overrated, though. Barriss had none when she kicked Ahsoka's ass and threw down with Anakin. Smh.
To be fair Atton Rand is quite under-rated, anyone powerful enough to be recuited by and work for Darth Revan's elite Jedi-killing operatives and for years at that has got to be very well skilled in anti-Force User techniques. He is also a highly trained practitioner of the Echani rituals and has strong danger sense.
Now place on-top of that the Exile's Force bond training and he would have become very formidable in combat. Traya admitted that his TP blocking techniques were impressive and from her that says quite a lot.