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Assuming you followed all instructions correctly, it should be possible to 1 shot at level 10-15.  If you only did 10%, it sounds like you weren't using the strategy.

Basically the way Elucia's skills work if you learned all the required skills:

Elucia gains a damage buff the lower her hp is. Euphoric seed reduces her HP to 1 and gives her crit and atk buff thus maximizing the buff. Amplify basically doubles her damage. Then Hero Strike does massive damage because  not only does it have a high damage value, but it completely ignores enemy defense.

The boss only takes a turn after all 3 characters have taken one, so if it gets a turn, you did it wrong.

Although I did forget that the MP passives were a thing, but yeah you need those on Elucia too. You also probably need them on Marie to be able to use Amplify 2x in a row.

Hero strike is a really strong skill, even without Ran, and just having Amplify on Marie, I could 1 shot Belobog the superboss in the forest at level 5, and that was way before unlocking the nun upgrade station to boost Elucia's attack.