Your only other options will be to fuse Epic dino DNA (such as the new Procera or EdmoG), open incubators with epic dna, defeat towers and collect epic dna, and last resort is buy your way through epic incubators.
This is one of the more tough tasks definitely if you have a small alliance or few active members or you get it as a task at Rank 4! Getting Daily Defense for 3 ranks is a kick to the face = that’s 195 arena incubators after defeating 10 creatures!
Our alliance has been working hard on the missions and have discovered the following information that can be found here: FAQ: Alliance Missions Coin/Cash/DNA Amounts and Tasks
We have only 2 items left to test and a couple to re-confirm once we start next week Alliance Missions.