The biggest chunk of guild coins comes from completing raids. Completing one gives you anywhere between 800 and 2000 coins depending on your damage dealt. All raids can be cleared in under 2 days. The only issue is being in a top guild to both get enough stamina for resets and good enough to clear the raids fast.
This really is the solution to guild coins. Join a top ten guild on your server. They usually are looking for active players. Each time a guild raid is cleared, coins are awarded. I get 1000 - 1.5 k each time due to ranking damage and a top guild can clear five or more raids per week.