Posted below is what I Posted on TERRA's Wikia page.
QUETZAL, UR (diamond, green) 32h. Breed: (DIAMOND & [FOREST or TERRA]) = 2 dragons. (C: FOREST, 20 seconds) or (UR: QUETZAL, 32h). My Breeding THEORY is that repeating the same pair w/out interruptions to Breed something else increases odds of obtaining UR dragons after each lesser Rarity Fail. Breeding QUETZAL to breed INFINITY are 2 of the "easiest" UR dragons to obtain [CRUSADER being most easiest]. 3 attempts for one lucky duck? Over 300 for another player? Luck & stubbornness is required.
After this Dragon Tales I will experiment Breeding w/ Elemental Types (ET). Anything you would suggest I try? I am REALLY interested in seeing if ET's have any special Breeding or Battling qualities. Will we be able to Breed ET hybrids?
I am hoping Wikia's Type Advantage Chart will be updated soon to include the types: Dark, Elemental, & Tarot. Though I have read, here & there, on the Forum, that none of them will provide any Type Advantage... Is this correct?
I am thinking you may have misinterpreted the info on the page. Terra x Diamond always results in Quetzal, likely due to an oversight on TL's part. Similarly Aqua x Diamond always results in Mercury, and, although untested, Ignis x Diamond most likely always results in Crusader. The Elemental Type's special breeding qualities are outlined in the Parents Behavior section on the Aqua Dragon and Terra Dragon pages.
As for in the arena, everything I've read/been told is that the Dark, Elemental, and Tarot types have no advantages or disadvantages in the arena. If a pure Elemental or Tarot type were to exist in the arena, they should be treated just like one would treat one of the corrupt dragons (except Corrupt Stalwart and Corrupt Artifact), or an Olympus Dragon, to a lesser extent.
The Breeding Calculator is incorrect (or else it has been fixed, but the code isn't showing yet). The info on the Aqua, Terra, and now Ignis pages are correct, at least for the time being.
As for the Impetus Dragon, if it were a dual type Elemental hybrid, it would act the same, but since it is a triple type, it would act like a hybrid dragon of the non-Elemental types (ie: Mistmoth or another Blue/Purple hybrid). Since there are more types to work with, Impetus is more stable than the other three. It could result in Mercury or Anubis when bred with Diamond, but it could also result in any Blue/Purple hybrid. And since there would be four types present, it wouldn't surprise me if it could even result in another Diamond.