Certain location spawn certain coromons.

while a little outdated, you can look up "Coromon Tracker" on google and you should be able to find a website that tells you where each coromons can be found in which patch of grasses


Another trick you can do, is using the randomizer settings back at the train station where you set your difficulty for the first time. That way you can sort of control the encounters so a rare coromon can spawn more commonly in another area (note that this will lock you out of the difficulty related achievements if you do this)