Veino Garn
Saaraai-Kaar
Veino's my most powerful FU, or was, anyways. I think technically that would go to Roth or Tiland now, but anyways! Veino was never immortal or a super-powerful Master. He could definitely have been beaten (usually was, actually) in fights, especially against other Force users. I can't remember him actually fighting against NFUs, but if they had good writers and thinking, it's not that difficult. The biggest things are countering lightsabers, disrupting connection with the Force, and staying out of range. A good NFU can do those things fairly easily with the right gear. Even a lot of blaster bolts in a scattered shape and high velocity are super difficult to handle. It's part of the reason Veino ended up focusing so strongly on defense, mastering Soresu and wearing somewhat-heavy armor.