Necropolis units are very weak, so your army in the final battle must be 3-4 times larger than the enemy's for a confident victory. Therefore, it is important to focus on increasing the number of any units, the main thing is to have thousands by the end. To do this, you need to follow a few rules. 

Win Without Losses

Easy to say, but in reality you just need to learn who to attack and who not, who to fight and who to let escape. This cannot be taught, but after a couple of hours of playing you will figure it out yourself. For example, you should be wary of archers - that's a simple and obvious advice, but also, if you have a large army, you shouldn't fight the Kittenhorn - it casts berserk on the unit that killed it. I lost half my army that way when I rushed in with tier 7 to crush them and didn't even realize what was happening. After that, I just avoided the Prismatic Nest entirely - the reward isn't worth the risk of losing half your army. 

Therefore, it's better to return to neutral units that pose a threat now than to lose 50-100 archers. Also, if the neutral army has archers and we are not sure we can kill them immediately, it's better to let them escape. 

Use Necrotic Energy Wisely

At first, you have a lot of energy and it seems like it will always be that way - but as soon as you start resurrecting 50-80 skeleton archers or tier 3-4 creatures, it runs out literally on the second day. So I started moving heroes in groups of 2-3; at the start of the week, the main hero fights with the army, then a secondary hero takes it, and so on. That is, I run two or three heroes with one army to avoid fighting when the hero's necrotic energy is at zero. Therefore, pass laws that increase the necromancy percentage and the amount of necrotic energy as quickly as possible. 

Some buildings are better not opened immediately, for example, Meareas Altar, guarded by a tier 6 creature, Quatl. It's easy to kill if you have 350-400 skeleton archers, one shot. But it's important to have a high necromancy percentage to have enough for resurrecting Dread Knights and their upgraded versions. On some maps, there are 10 of these altars; if cleared early, you only get gold, but if cleared later, you get 10-12 tier 6 creatures. 

Any Hero Levels Diplomacy

Diplomacy works very well here; with an army that can be assembled by the second or third week, almost all tier 1-2 creatures will join us. Considering that the Converter of Undead transforms creatures into the same tier, not just into skeletons as in the third game, such diplomacy allows you to greatly grow your army, especially in the late game when you have a lot of gold but almost no necrotic energy. 

At first, it may seem that the best hero for this is the King of Kings, but in general, any hero who has learned diplomacy is even better if it's not the main hero but one who runs alongside and takes the army when the main hero's necrotic energy runs out. 

Upgrade All Army Growth Improvements in Cities 

There are many laws that increase army growth - it's better to upgrade them as early as possible so that the total army size accumulates from the first weeks, and only then upgrade laws for army strength. Laws aimed at the economy are better skipped entirely - law points are too valuable in this castle to spend on gold or mercury. 

Also, be sure to upgrade recruitment and place a hero in the castle to further increase army growth. If there are two castles, it's better to have two heroes. 

Capture as Many Cities as Possible

Sounds stupid, as this advice can be given to any faction, but in reality, for other factions, if there is no money for development, it gives literally gold and a few creatures - possibly not of your faction. For Necropolis, they will always be creatures of your faction if you run them through the Converter of Undead. Therefore, increasing the number available for hire is best done from the first days.