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Lider1 is actually a map created by the author metatxer, and the aforementioned battle with the angel and the medusa is from this map. Like many others here, I played through the first day, but the checkpoint in the form of a pillar, which is supposed to be removed by a computer-controlled unit, didn't work for me either, so the map is likely unplayable in terms of the author's original intention. Recently, wenwell described the possibility of completing the map by exploiting a flaw and finding a flying creature, but that is, of course, not the author's intended solution.