https://gamefaqs.gamespot.com/boards/197341-final-fantasy-vii/80738220
Tl;dr - due to the way the Lucky 7s thing works, enemies who have these HP values:
7,777
73,313
138,849
204,385
269,921
335,457
400,993
466,529
532,065
597,601
663,137
728,673
794,209
859,745
925,281
990,817
Will hit for 7777 damage until their HP changes
Post # 8 has the relevant info. I didn't even know that enemies could get the Lucky 7 status >_>
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Oops, we trained a monkey to be a fascist!