Epiphanius must have started composing, or at least mapping out the Panarion*, or “Medicine Chest [against Heresies],” while he was still working on the Ancoratus. Ancoratus 12.7–13.8 (GCS n.F. 10.1:20–22) already lists the eighty heresies (twenty before Christ, sixty after) of the Panarion. The main composition of the Panarion took place roughly in the years 375–378. Like the Ancoratus, the Panarion was composed upon request: a certain Acacius and Paul had “heard the names applied by Your Honor to the heresies,” and asked for a full explanation of them.58 Whether they had read the Ancoratus and were puzzled by the (out-of-the-blue) list of heresies, or whether Epiphanius was publicizing his list by other means, we cannot know. Epiphanius composed the Panarion rather quickly (in under three years), considering its length, complexity, and (presumably) his other episcopal duties during this period. As is the case with the Ancoratus, Epiphanius's compositional style seems to have left little time for editing after his initial dictation. The Panarion contains a wealth of other Christian documents (from both “heretics” and "orthodox") our only Greek fragments of Irenaeus's Against Heresies, Ptolemy's Letter to Flora, and various documents pertaining to theological struggles during Epiphanius's own time, to name just a few. In addition to these texts, Epiphanius has also included writings of his own, dictated directly into the Panarion, including a lengthy “refutation” (elenchos) of Marcion's Bible, which he had written “some years before” (apo etōn hikanōn)59 and later embedded in Panarion 42 (against the Marcionites),60 and his aforementioned Epistula ad Arabos, in defense of Mary's virginity. These are merely the two earlier writings he names: given that he also copies material from the Ancoratus (sometimes without signaling that he is doing so61) we might also imagine Epiphanius economically recycling other earlier written materials at hand that have not otherwise survived.[Jacobs Epiphanius of Cyprus: A Cultural Biography of Late Antiquity p. 20, 21]The more I think about this and look at the evidence the more certain I - and everyone else - should be that the reference to 'possessing the Marcionite Bible' and carrying out the minutia of textual criticism was entirely false.
58. Epistula Acacii et Pauli 1.9 (GCS n.F. 10.1:154).
59. Epiphanius, Panarion 42.10.2 (GCS 31:106).
60. Epiphanius, Panarion 42.11–12 (GCS 31:107–82). Note that, although this refutatio is only two chapters of the Panarion, it covers 75 pages in the critical edition.
61. See, for instance, Panarion 70.7.6–8.4 (GCS 37:239–40), which repeats the argument of Ancoratus 54.1–7 (GCS n.F. 10.1.63, 64)
Look at the 'hurried' character of the Panarion as a whole now coupled with the fact that the outline against Marcion was never completed but was - paradoxically - carried out a while before. It would have been hard - if not impossible - to dictate this work. Rather I think he might have asked one of his scribes to go through previous works written against Marcion and cull as much useful information about their writings as possible.