Cool matchup. The Kothoga is relatively unimpressive when compared to the Deathclaw. While it's highly resistant to bullets, can tear humans to shred effortlessly, and as fast as a a lion, it still doesn't compare to the Deathclaw.
The Deathclaw regularly slaughters Brotherhood of Steel members who use heavy power armor. It has been described as killing an entire group of BoS members before they could even react. It shouldn't have a problem piercing the Kothoga's hide and with the speed difference it's a clear win.