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Nasos, There is a certain degree of variability, for example, in the Charlize branch. I first killed Synke (she can be killed without armor), and only then Inteus, which is why Inteus had a suspiciously easy time (the main thing is to find the initial setup where the phoenixes commit suicide against the azure creatures). However, the next battle against Fiona was very difficult, and all because the archdevil was sacrificed in the battle with Synke, and instead of him, I had a unicorn, so Fiona was killed by removing her retaliation and with a lot of randomness (the effect of fear).
In the Ariadne branch, there are also options; Dandy pointed them out. I will add that the battle with Thorgrim can be passed with
walls without blindness. More mana, but it's not a pity to spend. The battle in the Ellen branch with taking Pandora (dragons and elementals) is also noteworthy in terms of options.