Thanks. My favorite Genre is Science Afflicted Fantasy, which is my name for magic that still obeys science. I spend a lot of time thinking up things like that. A couple other ideas are:

Earth Element:
Create a rock blade, then force the molecules to vibrate at high speed. The resulting weapon uses up a lot of your energy, but can cut through nearly anything.

Fire Element:
When Confronted by an Earth user who uses the ground beneath your feet as a weapon, find a dry area and turn up the heat. If you're lucky, the ground you're standing on is rocky and turns to magma, which you can keep the other elementalist from controlling. At the very least it becomes scorched and barren, probably hard for an earth user to control.

Water Element:
Like the earth element, you can use high preassure water streams to cut through most solids. The higher the pressure and thinner your stream, the more cutting power you have.

Air Element:
By stopping the air molecules from vibrating, you can create an area that sound will not pass through. further more, if you're strong enough, the same usage can create a solid wall of air, stopping anything from crossing.