How do I complete the final mission playing as Roland?

I don't really see any problems with this approach. You pick a Knight with high Wisdom, upgrade Giants to Titans, and with 5 Titans and the Crown, you can reach day 11-12 as a Knight and defeat the Warlock. Then, you capture everything, buy Dragons on day 15, and clear everything around, including the Barbarian and Necromancer. You can also use a Sorceress with Phoenixes and Archers as support, which works well against the Necromancer. This isn't the best strategy, but it's quite effective.