Each time Challenge 1 in Challenge Of The Titans mode is successfully completed, you will get one Cyclops Eye. You can repeat this as many times as needed.

Unlock the Arena Of The Fates, then make three Cyclops and have them respawn and not attack to get twenty Cyclops Eyes quickly.

When the Bowmen (little penguin-like creatures with axes) call the Cyclops with their horn, just kill the Cyclops and not the Bowmen. After you rip the Cyclops' Eye out, the Bowmen will call another Cyclops. You can keep doing this until you get all twenty Cyclops Eyes.