This guide will provide you with the fastest and easiest way to farm any rare monster rainbow eggs.
Step 1: The first thing you will need to do is to pray or offer to find a charm to the prayer pot.
Either one works, they do not stack.
Step 2: Check Monsterpedia for desired monster Habitat.
Step 3: You have to press RT and then press Y to fast Travel there.
Step 4: Ride out with a Monster with Nest Searchability.
Step 5: After that, you have to check the map to see the rare monster den location.
Step 6: Reset Dens by fast travel to the same location.
Repeat this step until the rare monster Den appears. You can press a button to use the monster ability while riding. Any Monster with a nest search can do this, even Kulu.
Step 7: Head into the rare monster den and press A to use the nest search again to quickly locate the egg. Make sure to grab the ores, they could be rare charms.
You only want eggs with gold/rainbow light effects.
Step 8: You will need to repeat steps 5-7 until you get what you want. You can even get a new Gigante Egg with this method, without unlocking the crystal depths.
Step 9: Enjoy all your rainbow eggs and open them up to get the rare monsters.