The “Maahir’s Gambit” scenario.

Hey everyone! I'm playing Heroes for the first time, and my friends suggested I start with "Gambit." I've been trying to replay it in different ways for the past five days, but in the end, the Necromancer always beats me. If anyone has some free time, could you give me some advice on how to better organize my army or what I should focus on? I have two artifacts from Sar-Iss, but I still need to find the others. I also have three additional heroes leading squads, but I still can't manage to defeat him!!!!
HELP!