Arcs correspond to a major part of the story. In the case of Re:zero, that would be solving one of the death loops. Arc 1 is the first day, arc 2 the dog attack, arc 3 the cultist attack, arc 4 is the sanctuary, etc.
Each arc is covered by a different manga series, from different mangaka. So you would have things like arc 2 volume 5, arc 3 volume 2, and so on.
Natively (a website that tries to estimate the difficulty of native content) has an entry for arc 1 and arc 3:
Both are estimated around N3 (that’s what level 25~26 means)
進撃の巨人 is level 29, so yes, they would be slightly easier than that.
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