I've been to these temples in the game that have spirits tell me how to get ultimate fire, dark, etc. I've gotten down you have to breed certain monsters to get the final monsters of each type. Here's the descriptions I got plus what I could figure out as the monsters fitting the descriptions so far
Ultimate Water: Giant Fish: Aquastorm Blade of Dark: Pencilhead
Ultimate Fire: Countless Spirits Beast of the Volcano
Ultimate Plant: Strange Fruit Giant Rock Elephant: Elevil
Someone else posted the fire, water and plant variants. Problem I found, you can do something like breed the fire+water final monster together to make an even MORE final monster. A fire/water mixed moveset with a spirit type monster.
Got some results from breeding. It seems you get supreme monsters from the combinations. For Ultimate water you get Orcanator And for light and darkness it's Fluttermus and Purpleeze. I even bred those two and got an extreme spirit monster with light/dark attacks called Chicklox.