Monster Combination

Air:
Air + Dark = Earth
Air + Earth = Air
Air + Fire = Water
Air + Holy = Fire
Air + Water = Holy

Dark:
Dark + Air = Earth
Dark + Earth = Dark
Dark + Fire = Holy
Dark + Holy = Water
Dark + Water = Fire

Earth:
Earth + Air = Air
Earth + Dark = Dark
Earth + Fire = Fire
Earth + Holy = Holy
Earth + Water = Water

Fire:
Fire + Air = Water
Fire + Dark = Holy
Fire + Earth = Fire
Fire + Holy = Earth
Fire + Water = Dark

Holy:
Holy + Air = Fire
Holy + Dark = Water
Holy + Earth = Holy
Holy + Fire = Earth
Holy + Water = Air

Water:
Water + Air = Holy
Water + Dark = Fire
Water + Earth = Water
Water + Fire = Dark
Water + Holy = Air


If you want the other way around where I start with the success here is the list:

Air:
Air = Air + Earth
Air = Holy + Water

Dark:
Dark = Dark + Earth
Dark = Fire + Water

Earth:
Earth = Air + Dark
Earth = Fire + Holy

Fire:
Fire = Air + Holy
Fire = Fire + Earth
Fire = Dark + Water

Holy:
Holy = Air + Water
Holy = Dark + Fire
Holy = Earth + Holy

Water:
Water = Air + Fire
Water = Dark + Holy
Water = Earth + Water