Skills for an Inferno hero.

Luck with Gatekeeper, Leadership with Gatekeeper, Attack with War Frenzy, Logistics with Gatekeeper, and Dark Magic with all spells – this is an unbeatable build for a Demon. Chaos Magic will only be good on small maps, and even then, not always. When fighting against superior enemy forces, only Dark Magic, no other options. And if you're also lucky with the Call of Urgash spell, you can wipe out everyone without any losses. Ballistics is a useless skill for a Demon on large maps. Even if it's a Daleb. It might hit a stack a couple of times, but then a smart opponent will destroy it, and the entire skill slot is wasted. Against legion-sized armies, both Ballistics and Chaos Magic are a pointless waste of time and effort. Defense is also useless for Demons. If the enemy attacks, there will be heavy losses anyway, no matter what. It's much more effective to simply prevent the enemy from attacking.

I agree with everything, but:
Ballistics can be useful even on large maps – it depends on the availability of a mentor and money. If there are no money problems on the map and the mentor is nearby, Ballistics will make life easier in the early stages of the game. Since the opponent runs around with a small army, and it's difficult to destroy the Ballista's hit points + it will be effective against early-game mobs with its 3rd shot (especially against archers and mages with 7 Imps). It's not the best option in Multiplayer, because if you split into 4 stacks, you're guaranteed to lose, and that's a waste of money on ransom – although it might be worth it. After a month or two of playing, Ballistics will no longer be needed.

But on huge maps, education through mentorship can be useful. Simply to level up all additional heroes to around level 25. With Berserk or Subjugation, these heroes can harass the opponent well with minimal losses, but this is more for solo play.