I render everything in my game in 2D with SpriteBatch with the exception of a spherical entity (orb) that I render as a spinning and dynamically lit (custom effect) sphere in 3D. I capture the sphere into a RenderTarget every frame and then use the image in the SpriteBatch to render in 2D. The composed image is aliased as any model.
I do not enable MSAA because various platforms that my game runs on (WinRT, as an example) do not support it. I tried FXAA as a custom effect, but the problem is that it does not appear to support transparency (I render the sphere on a transparent background).
My rudimentary solution is to render the sphere into a RenderTarget of size X and then render the result with SpriteBatch into a RenderTarget of size X/2. I set the SamplerState for the SpriteBatch draw call for the half size rendering to anisotropic, although I am not 100% sure this increases the quality of the final output dramatically (and at what cost?).
This solution appears to work fine but is not without its flaws. Mobile devices are slowed down by the multiple SetRenderTarget calls per frame, especially since I render more than one orb type.
Are there other approaching to this?