Walkthroughs for custom maps.

yes, I also periodically experience nostalgia for offline tournaments, and therefore from time to time I play through maps, but

What is called "hardcore" is, in most cases, just a fancy word for those who like to play with heroes while drinking beer)))

we need to fight against such stereotypes. I would immediately suggest
1) Metataxer's Revenge http://heroesportal.net/maps/view/3850 without hints, but some people would consider me a sadist. The map can be completed in just a couple of months, by the way.
Marlo, for example, would probably suggest Xeidu and Limes Inferior, but they are not faster. And the second one is not very difficult.
First blood is the map that received its Walkthrough in 24 days according to Anton's ideas, with the implementation by Ksenia Sobchak. But we will talk about something else:) Anton once suggested a map for completing in one day with a lot of interesting features to test your understanding of the mechanics. But over time, a more complex map appeared to test your understanding of the heroes' mechanics, so
2) Valley of Heroes by Rosekavalier http://heroesportal.net/maps/view/3939
A puzzle map, can you complete it in one evening while drinking beer?
3) Rise of the Sun King http://www.maps4heroes.com/forum/opinions.php?map_id=1211&game=3 by the author of Metataxer's Revenge. It is currently impossible to complete, but in this case, it is suggested to simply play the first day.

All the previous maps have problems with taverns, and you need to rely only on the heroes provided by the authors. However, you should not think that the issue of managing turns is elementary there.
Perhaps you want a more familiar offline challenge, with taverns and hero purchases. There are simple maps for this, but they have difficult tasks that are limited to a small part of the map.
4) Limes Inferrior http://www.maps4heroes.com/forum/opinions.php?map_id=1537&game=3 - you just need to build a capitol and troops 1-6 by day 147. What? A whole month for building troops, and you call that a worthy offline task? Well, not a whole tournament, but yes, it won't be easy. The only note is - don't give up the quest artifacts, as it is assumed that you will continue playing)
5) Perhaps you have heard of the well-known map in narrow circles, Paragon http://heroesportal.net/maps/view/3480? The map itself contains the Admiral's hat, which hints at great difficulties in checking various speedruns. Therefore, let's consider everything that is not prohibited in the HD mod to be allowed. And the task is simple - get the Admiral's hat on day 117.

For all 5 points, for playing at 200%, it is suggested to deal with the generation of starting conditions yourself (and this is important in some cases). And if they do not require sobriety and complete concentration, then it should not be a big problem for you to complete at least a couple of these tasks.