(THIS IS NOT MINE. JUST AN TRANSLATION of @Very Wells guide. All credit goes to @Very Wells)

In the reviews for the game and on the forum, I read complaints about the difficulty of leveling, even that the donation does not help to progress faster. You're right, a donation will help you upgrade part of the game, but not all of it, then you need tactics, not donation.

For the first week, I myself was at a loss: why did the game stop progressing at one point and does not move further, although I have studied almost 90% of the contracts and it seems that there is nothing to improve for progress?

This is actually the mistake of most players who have played Clicker Heroes 1 or Adventure Capitalist or other similar games before. It's the same pattern everywhere: on the left, you're given heroes that unlock gradually as you progress. Each time you unlock the next hero, you need to invest more resources and power-ups, which you a) accumulate over time, b) gain through constant prestige.

That's how idlers work: day, week, month, you're constantly making prestige and moving on and on until you've unlocked all the heroes and finished the game.

But the problem is that the rules in this game are different and playing according to the above scheme you will never finish the game.

In fact, everything I've described above concerns only one faction in this game - the first faction - the demon faction. That's how you should play this faction: constantly make prestige, here it's called boiler explosion - for prestige study contracts, making demons stronger and stronger and unlocking more and more demons in the next runs.

The main rule in leveling up the demon faction is that you need to determine the most effective boiler explosion speed for you personally. The minimum amount of ash for a boiler explosion is 75. Write down the time it takes you to collect 75 ash-90 ash-110 ash and so on. divide the amount of time spent by the amount of ash received during this time and choose the FASTEST way to farm ash per unit of time. And use it constantly, every day, until you level up all the contracts.

This is where the similarities between this game and other idlers end.

Because in the game, in addition to the demon faction, there are at least 3 more factions: undead-drowning-warlocks.

And these factions will NOT develop according to the demon scheme: you invest maximum resources in the lower hero, the next hero is unlocked. NO, this is not how these factions work, according to this scheme you will stop progressing.

EVEN if you level up all the contracts to the maximum, you will not be able to advance further in the game even a millimeter.

Because the rest of the factions develop through certain tactics that involve step-by-step progress towards the goal.

There are different rules here:
1) You can't use all the resources to the maximum, as you did when leveling demons. For example: if you level up the first hero, Leviathan, to the maximum of resources in the drowning faction, you will simply stop progressing and will not be able to open the dark obelisk in the undead faction. Because you won't have anywhere to get the huge amount of waste you need for this task.

2) Starting with the second faction after the demons - the undead faction, and further, by other factions, carefully analyze the tasks ( building a dark obelisk for the undead - access to the depth for the drowning - unlocking the warlockIn this case, it is important to note that this is not the case. For all these tasks, you will need to understand how to accumulate the necessary resources in the right amount.
Each of these tasks is performed in a different way, but all of them are related to the correct accumulation of resources that are spent on hiring faction heroes.

Each of these tasks is a separate task with its own tactics and ways to achieve the goal. Each of them is a test of ingenuity and understanding of the rules of resource gathering in this game. You won't be able to buy heroes with all resources, like in the demon faction.

In an adjacent guide, I detailed how to build a dark obelisk with the undead faction. There you will be able to understand more how the leveling of the first faction, demons, differs from the rest of the factions.
For example, 2000 manual clicks to hire 2000 red herrings are worth it, because the autoclickers in the game do not work, and mechanically you can buy either one herring or the maximum amount of herring. Buying the maximum amount of herring will instantly destroy your progress in building the Dark Obelisk. And there are a lot of such nuances.

All this makes the game unusual, interesting, not like the usual monotonous idlers.