The Obsidian Dragon can be obtained by breeding an Earth Dragon or earth hybrid dragon with a Fire Dragon or a fire hybrid dragon. Its breeding time is 8 hours, or 6 hours 24 minutes if you have an upgraded breeding cave or island.
Bear in mind that breeding an Earth Dragon or earth hybrid dragon with a Fire Dragon or a fire hybrid dragon can also yield a Lava Dragon or, before Halloween, a limited edition Bone Dragon, both of which have breeding times of 10 hours, or 8 hours with the upgrade.
As for the best combination, breeding in DragonVale relies a lot on randomness. There is no canonical "best" combination for most dragons. No one seems to know for certain the odds of each outcome from every combination. However, according to this DragonVale Breeding Sandbox, the best combinations (without an Obsidian parent), with a success rate of 29%, would be:
- Bone and Mud
- Bone and Water
- Fire and Mud
- Lava and Water
Considering some of these combinations, it would seem all you really need is to have the earth and fire elements.