PReDS
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Based on the arenas I've tried, it's always a one-sided game. The only creatures that have a chance against my skeletons are the aastr elementals, but their chances are very slim: they don't have the Ring of Protection, and my skeletons can take down the summoned stack with a single attack. In the other arenas, there's no need to come up with any strategies.
P.S. Theoretical discussions about the task are good, but I suggest asking Pred to share his walkthrough for HeroesWorld, as he seems to have completed the task better than anyone else in practice.
As for the missing arenas, MantikOre yields the same result. In the arena with Dust, everything comes down to who gets hit first. The fact that he doesn't have liches (which means his skeletons move before mine) gives him an advantage in maneuvering, and I get hit first. Combined with Dust's wider selection of spells, his advantage is undeniable.