It’s worth noting that you only need to defeat the Barroth in the Arena in order to have a chance at successfully getting one of these coins.
If you’re trying to get you some Monster Hunter World Hunter King Coins, we recommend taking on the High Rank Barroth repeatedly in order to farm for the Hunter King Coin. That monster is the Barroth, specifically, the High Rank Barroth. There are actually a few different beasts that you can fight in the Arena to get the Monster Hunter World Hunter King Coin, but there is one that we believe is both the best and easiest for doing so. Monster Hunter World Hunter King Coin Guide: How to Get This Coin From the Arena