Her match scaling is the same as the difficulty scaling (1.5, 2, 2.5, 3, 3.5, 4, 5, 6, 7), so at her 9th match, assuming Difficulty 0, she'll deal ~34,000 per hit at most, ~101,000 with crits. You probably want your MHP to be double those values, roughly. Her AGI maxes out at 2,030, so that's the number to beat. At Diff 9, those values become ~235,000 (~705,000) and 14,210.
MAG is probably the better attacking stat to pump here, because Miranda/Meryphilia takes double damage from light, it helps with MP drain (Her MMP is Diff 0 - 700, Diff 9 - 4900 on her 9th round, it isn't very high), and it helps with healing (though items are a better source of heals).