Islands of Secrets

Could you please advise on how to complete the 5th mission, "Peak of Stars"? I always play as the hero, and the first four maps are indeed very easy. The 5th mission starts well, but I can’t even last a week. Three to four demon heroes attack in succession and defeat Geralt no matter what, both inside and outside the castle. The distraction caused by the archers is especially annoying. Could you suggest how to defeat them? Am I missing something? Geralt has the following basic skills: 7-5-2-2-3-0, level 11, and the following advanced skills: Expert Magic (Gift of Protection, Gift of Blessing, and Guardian Angel), Expert Offense (Tactics and Archery), and Expert Leadership. His army consists of 5 Knights, 10 Swordsmen, 116 Archers, 10 Royal Griffons, and 15 Peasants.