MjTh
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Why is it not easy? In a month, you can accumulate enough resources to build all the guilds (although this is usually done towards the end of the construction).
Dark Magic is strong, I agree, but it can be dispelled, and it deals minor damage.
And why do you think that the Inferno army is not the best? In my opinion, it is on par with the human army and ranks among the best.
Dark Magic is strong, and it has some features.
1. Use all spells of levels 1-3 quickly (be sure to take "Spell Power", preferably "Keen Mind").
2. A very good tactic is to rely on the Pain Lord: especially when the enemy has many strong units. Destructive Ray will do the trick + the opposite effect of Stone Skin only increases by 12 at max. If you also pump up Dark Regeneration, it will be easier to compensate for resistance.
3. Play 1-on-1 with spells. In principle, there is not much benefit from the magic itself; when the enemy casts Haste, you cast Slow, but he does not gain a significant advantage, and if he stumbles somewhere, it will be easier for you.
Do not risk taking Chaos for the demons; you will waste a skill. Sorcery is not the best choice, unless you have many planetariums + Education. You can also pump up a lucky sorcerer, but the best option for a Demon is still light, darkness. Chaos is very powerful for Undead, Necromancers, and Mages. Dwarves are better off pumping up Summoning.