I finally got around to testing this battle with higher stats, in order to settle the debate once and for all over how much agility actually matters with regards to Mastema getting first strike on Master difficulty. I used a save editor to alter the levels and stats of Flynn and several demons, boosting them higher than any player would ever realistically have. Flynn now has 200 Ag, while the demons have 400 Ag (which, as it turns out, doesn't seem to matter at all in terms of first strike odds).
So, how much does 50 levels and 130 more Ag on Flynn help? Well, instead of getting first strike 100% of the time, Mastema only gets it 50% of the time. I ran 25 trials, and he struck first in 13 of those. So in conclusion, Master difficulty doesn't actually care that much whether your Ag (and possibly level) is high or not. It cares 50%. Taking that into consideration along with his scripted opening turn of two Megidolaons, I have very little faith that the developers playtested this battle (along with a good portion of the rest of the game), because it's idiotic that the odds of dying before getting a turn would be that high.
As a side note, apparently Mastema's scripted attack sequence on his first turn (Megidolaon 1, Deadly Wind 2, Megidolaon 3) gets screwed up if his first Megidolaon misses. However, that's never actually going to happen unless you have demons with absurdly high stats like I was using (which is to say, hacked or after a ridiculous amount of incense farming), so it's completely irrelevant.
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