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It's a thankless task to play against Necropolis...
As an alternative, I suggest playing as Rampart. The Necropolis specializes in melee combat. In this case, it's best to immediately use an expert-level slow spell on the entire enemy army. Necropolis units have limited access to mass dispel, so the next task is to neutralize the Liches. You have all the necessary tools – 3 ranged units. If all the Liches aren't killed, they will be severely weakened. In the first round, the Rampart army shouldn't engage in melee combat (unless the Dragon Ghosts fly in on their own to their doom). Use the Djinn to cast some beneficial spells on the Titans or the Nagas – these units are very important in battle.
In the next turn, the initiative is ours again – this time, focus ranged fire on the Vampires – they don't have that much health, and the Titans and Mages should be able to handle them. The strength of the Vampires increases disproportionately with their numbers (20 Vampires are stronger than 10 Vampires, not by a factor of 2, but probably by a factor of 3-4). It's important to reduce their numbers as quickly as possible.
Use combat spells – it's better to use an explosion on the remaining Vampires and Liches (if many survived) or, as an alternative, use a mass haste spell on your own units. Combined with a slowed enemy, many can simply run away from the horde of skeletons and shoot from a distance. The final blow should be aimed at the Dark Knights – by the 3rd round, they will already be engaging in melee combat – it's important that their attack doesn't hit the Nagas, Mages, and Titans. It's best to use Golems as a shield – there are many of them, and they are very durable level 3 fighters. Golems are good against Vampires (they won't regenerate, bloodsuckers!). I would like to point out that the skeletons will only reach you in the 4th round of the battle if you maintain a defensive position! By this time, the main striking force of the Necropolis (levels 4, 5, and 6) should be suppressed. The Dragon Ghosts are weak and, without support, are unlikely to do anything significant.
So, by the 4th round, only a horde of skeletons, zombies, and possibly ghosts (if they didn't engage in direct combat earlier) should remain against you... the rest is just a matter of technique...
although I haven't taken into account the opponent's ability to cast spells – but there are so many options that it's impossible to predict everything –))))))))
And one more thing: when playing as Rampart, if the Rampart army and hero are generally stronger than the Necropolis (except for the skeletons), you can try to tie up the skeletons with Dryads. Considering the number of Dryads and their health, they can hold the skeletons in one place for some time while the rest deal with the enemy...