I think you're a good level for around that part in the game (it's been a while since I played IV though). If you cast Slow on the Demon Wall it should give you enough time to buff yourself and destroy it. I think I beat it first time using that strategy. Come to think of it, cast Slow at the start of every Boss battle, because I'm sure most of them aren't immune to it.
I hope that helps you (at least a little bit), and don't worry too much about not giving the twins 3 Augments each, you can go back and do it again with new game+
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