Battler didn't have an explanation, but he still wouldn't accept magic as existing. So now Beatrice will show him a reality with (what appears to be) tons of magic to try to stump him.
Just because we (and Battler) are seeing certain things doesn't mean they are what is actually happening. You can decide to be anti-mystery and argue that it is, but Battler's position is that it's all a deception and that he can and will prove that what really took place was done through human, non-supernatural means.
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