What is your unit setup? I do Pro with any lightning over 500ATK and it takes 2-4 turns, depending on how high that atk is.
My setup is a 6* Pecil, chizu with murasame, Vaan with enhancer, kefka, lenna.
cheer and focus first turn with pecil and lenna, vaan full break, chizu barrage, lightning crushing blow, and kefka hyperdrive.
Each subsequent turn Pecil can use drainer for a little damage, Vaan has barrage (but could also use full break; he doesn't do much dmg regardless), and lenna can either heal or attack.
The majority of the damage is coming from hyperdrive, crushing blow, and barrage. The rest of the crap is just for chaining to get those 3 higher.
edit: It's worth mentioning that any character over 500 should work so long as they have DW plus barrage or another high-damage attack. I assume you have at least one or two friends now with DW, excal, and barrage?
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