Chapter 7 - Act 4 (Breed Snowhound Dragon with another dragon)
Short of skipping this act, there is
NO WAY you are going to avoid the possibility of breeding a 25-hour Penumbra (Blue & Purple) Dragon, because Royal may split into Purple and Yellow, while Black may split into Purple and Blue.
You may, however, reduce your chances of getting the Penumbra by breeding the Snowhound Dragon with pure Yellow or Green so that if Black does not split, then Penumbra will be left entirely out of the reckoning. Avoid Red, White, and Pink, or any elements that might split into White or Red, as these will bring in the Titan (32-hours) and the Moon (31-hours) into the possible outcomes.
If you are feeling incredibly lucky, you might wish to try breeding with a pure Diamond or Diamond hybrid for a chance at the Mercury Dragon, which has a breeding time of 1 hour
Of course the other Diamond hybrids have breeding times that range for 24 to 44 hours.