How do I complete the final mission playing as Roland?

It all depends on the troops. If you're relying on Titans, then returning (not necessarily a true return) to them is a good strategy. Cloning also works well. Earthquake is unlikely to be effective – Titans have high defense (15) + the hero's defense, so towers won't deal very high damage. Casting "Slayer of Dragons" is a good idea, but it makes sense to use it when the damage dealt by the unit is significant (for example, on a large number of Minotaurs, or of course, on Titans; it can also be used on Archmages if their griffons don't block them).