Hero engineers.

In the original version (3.0), machines are a real headache. Every faction can get machines with a triple ballista firing fire arrows as early as the first week. All it takes is a level 7-9 hero. Demons are slightly more challenging (levels 12-14), but that's understandable, considering that demons grow primarily in attack and knowledge, making their ballista the strongest of all. After that, it's just a matter of technique: protect the machines with a tent, and invest in defense and logistics. An Orc with 10 green wyverns can easily defeat any monster, including Utopians. Of course, taking down a ballista in the late game isn't difficult; there are plenty of options, such as Deadly Cold, Mark on mages, and enhanced spells from the Dungeon faction. However, when it comes to clearing the map, the machine-focused strategy is quite effective. Perhaps a Warlock with invisibility and hydras can also move quickly, but even that is questionable. If the map is small, the machine-focused hero can handle it on their own. If the map is large, they can control the map and the economy, and from the third or fourth week, the main hero can start leveling up. There are many options, such as Ilfina with regeneration and dragons, Nur with yellow golems and Destruct, or Mephala with archers. This is because if there's a lot of combat in the late game, the machine-focused hero won't be very useful; the ballista will simply be taken down, and the base will become dead weight. Unfortunately, it's a rather one-sided strategy. In the original 3.0, few heroes can match the machine-focused hero in terms of clearing speed. That's the sad truth.

and of course, the chance of getting a ballista is low

Why is it low? Initially, "Tremors" and "Rune Machines" are unavailable, the tent is always taken, and then advanced machines become available, with a 50% chance of getting a ballista on an adventure. Unless, of course, you choose other bases, such as logistics and defense, etc. Otherwise, the skills will naturally be affected.