User blog comment:Wrath022/Chapter 517: Wendy Belserion, Review/@comment-3290324-20170115062717/@comment-7418318-20170117154021

By dual wielding.

After all we see both Ikaruga's sword and one of Erza's blades crumbling after the clash, but Erza was still holding a second sword. You can't cut someone with a broken blade, so it means the cut was done by Erza's surviving blade.

So basically Erza and Ikaruga crashed their blades so hard together that both swords broke, but on account to Erza having two swords, she still had one left to cut Ikaruga down with.

I suppose withdrawing all her magic from defense and pouring it solely into her weapons, as Erza said she did, was the reason why the destroyed sword was durable enough to achive mutual annihilation with Ikaruga's blade instead of just being cut through like all the weapons that Erza tried to use before that point.