If you want to break rocks which are often obstacles to block various passages ways and loot, you will need a monstie with a rocker breaker ability. Monsties with the rocker breaker ability include the following monsters.
- Yian Kut-Ku
- Blue Yian Kut-Ku
- Sand Barioth
That being said, if you are looking to get a monstie with rocker breaker fast, the best one would be Arzuros, Yain Kut-Ku, and Blue Yain Kut-Ku, as the other monsters are unlocked later on in the game.
You can also find both Yain Kut-Kus on Hokolo Island inside both monster dens and roaming around. For Aruzros, you will need to progress to the second area in the game, and they can be found in a similar situation as the Yain Kut-Kus.
Meaning you will need to find them either in a monster den or roaming the overworld. When you do encounter one of the monsters, you will need to force them to retreat by breaking all of it's parts so you can get an egg drop that is guaranteed.
You can also increase the chances of the monster retreating by throwing paint ball at it when it is ideally at low health. Once the monster retreats, follow it back to it's den and grab the egg and hatch it - after that you will be able to break rocks with the monstie.
Check out some of our other Monster Hunter Story 2 guides!