The thing with final monsters:

You know how you can get breeding hints if you interact with the spirits in the Destiny and Spiritual temples? Some people think it's how you breed final monsters but actually the spirit tells you how to breed a supreme monster.

For example, take this sentence: "when countless spirits rest within the beast in the volcano, ultimate fire shall be attained."

The spirit is trying to tell you how to breed a supreme monster, which in this case, is Colthorn. You breed Bubblonster with Firefury to get Colthorn, which completely matches the spirit's information, because Bubblonster looks like a whole lot of faces morphed together while Firefury is fire type and the boss you have to defeat in one of the quests.

If you breed certain supreme monsters, you will get the four extreme monsters. If you breed extreme monsters together, you won't get a final monster, unfortunately, you will only get a plant-type monster, (and I don't mean the final plant monster Bucavactus) due to the fact that two spirit monsters make a plant-type monster. Let me list the combinations for you:

Preycillia + Lushtyr = Evilseed
Preycillia + Volcanoblin = Flowerpower
Preycillia + Chicklox = Plantagon

Lushtyr + Volcanoblin = Melatrocity Plus
Lushtyr + Chicklox = Botocurse

Volcanoblin + Chicklox = Evilseed

Sad, huh? That you obtained these very powerful monsters and you just get these common ones when you breed them together.