Aqua picked up on the fact that Signum was trying to avoid her lightning, so with a couple more strikes against her sword, she powered up to Thunderbolt style (http://kingdomhearts.wikia.com/wiki/Thunderbolt).

"Maybe you won't tell me what's going on, but I'm not going to let you do this," she said.

Thunderbolt would only last a little while, so she tried to make it count, meeting Signum's attacks with lightning-powered strikes. If she was lucky, she'd be able to stop her without causing any permanent damage.