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Let's revisit this topic.
In general, I rarely play as Inferno; it doesn't appear often in random games, and there are more worthwhile Castles to choose from. I decided to play a game at 160% difficulty against 6 AI opponents (not allies), and I won after about 3 months. My tactic was as follows: I chose Octavia, with the bonus of gaining money. As someone mentioned before, Octavia is a decent starting hero because she can be developed very well.
Day 1: Demons (Goghs were already built).
Day 2: Guild of Mages.
Day 3: Efreet.
Then, I eliminated all the neutral creatures (I got a Calphay in the tavern, so I didn't have any problems with ranged units). On day 4, I recruited Gnomish Goblins and a minimum-level Consul. I maxed out Octavia's Offense, Earth Magic, Tactics (which is very helpful when using Efreet against neutral creatures, especially if you don't have Mass Haste), Logistics, and Intelligence (it turned out that I couldn't build magic beyond level 3 due to the poor resources on the map, so the mana, given the low cost of spells, was enough for a long journey). I didn't focus on Demonology because I was able to survive without it, but in online games, there's no other option (I lost to Rampart in the third month playing as Inferno on an XL map).
By the way, the map had an Efreet dwelling, so the reinforcement also included unupgraded Efreet, which, with this tactic, allows you to reach the battlefield from the first turn. Since Inferno is an offensive Castle, I believe that Tactics is an ESSENTIAL skill because without it, even the Dogs can only stand on the battlefield.
In the beginning of the third week, with a squad of 8 upgraded Efreet, 26 Demons, 20 Cerberus, and 100 Imps, I defeated 80 upgraded Monk neutral creatures guarding the Celestial Helm (+6 to everything), and after that, it was just a matter of technique.