For those Epic monsters without opposite elements: You just have to use basic uncommon monsters to get it, and the chance is 30% for the single Epic monster.
Example: Utochomp and Shock Turtle => 50% Epic monsters In the double method, you will likely have a success chance 50% (total 4 Epic monsters). The first monster has 2 elements, the other monster should have 2 elements that are all opposite with first monster's elements.
For those Epic monsters within opposite elements: "Double chance" is the most effective way to make 4 Epic monsters by using 2 pair of opposite elements.
Example: Utochomp and Sheluke => 25% Epic monsters In this way (1 pair of opposite elements), your success chance is ~25% (2 Epic monsters - same elements).
Normal/classic way: breeding 2 common/rare monsters, which can combine 1 pair opposite elements (each element from each parent monster).