You're right that erza red pants armor don't have any magic but you're wrong if you think that her attack didn't use magic. She use it, the magic is in her swords.
Yeah ikaruga did said that the armor don't have any magic but it's said that she puts all of her magic in her swords to make a powerful attack.
It's later made clear again the power of red pant armor in her fight with azuma
So basically Erza red pant armor attack is Erza strongest attack after Nakagami starlight and this attack did basically little to nothing to the berserker as we can see here
Well, I think even with immense magical power as you implying... Ifshe still can't do anything. It's still no use.
100 Berserkers is too much for Erza.
P's. Accidentally deleted my post .