I just finished playing this mission. I didn't have any problems defending the castle. I didn't buy the native hero; instead, I bought a Warlock and started leveling him up in magic. So, I easily repelled all attacks with the hero's magic and the druids' magic, along with archers and ents. And I ran around with Fidaen and almost half the map with only dragons. I cleared the entire map, visited all the upgrade locations, and completed it in 3 months, 4 weeks, and 6 days. I could have done it faster, but I decided to visit the upgrade locations with magic guilds using the freed heroes. Now, I have to chase Tieru... again.
They probably mean the native hero is better because he can add someone to the list of enemies, so morale doesn't drop. And by the way, doesn't the Warlock have a -2 penalty? Then he needs to take Skilled Leadership to avoid this, IMO. This trick works well, for example, when you have a Knight hero, but you take the Fallen Knight skill for some reason, so he can wield powerful Dark magic.