Although Feln is huge and wildly impressive (almost the coolest lookin' thing in Star wars) Kas'im would probably still win. He was described as probably the best duelist up to his time and he killed a rancour so he's used to fighting stuff bigger than himself. Also he did well against the 'mountain of muscle' Bane, only losing becuase of Banes own force Behemoth status.
Kas'im takes the cake.
And the plate.
And your house.
'Nods' As well as your mum. 😉
But seriously, I don't know enough about Feln's force strength to make anything but an educated guess, but Kas'im gets it becuase although he hasn't shown us much, he was able to block a force push that had the power to shatter every bone in his body and essentially turn him into fleshy soup.