Some quick tips other than the great stuff people have given already...

Don't be afraid to use your ISO. Unless your blowing that picture up large, your photos should be able to handle ISO 1600 or higher if you expose to the right. Noise cleans up will with the T4i sensor. I have used the 70-200 f4L IS for frog pictures, but usually have a 1.4x TC to increase the distance and magnification. If you don't think your getting the right spot in focus, try snapping off lots of shots with your body sway. You can focus stack it later. In all seriousness, don't be afraid to experiment. Brace yourself on something so you have a little stability and snap away. I tend to not get higher than f/8 when shooting macro. I prefer to focus stack.

For diffusion, this might be something quick to make if you didn't want to spring for diffusers. Pringles Can Diffuser or Coke Can Diffuser. I made one out of a styrofoam cup and it works pretty well. I would set your flash to manual mode and set it to something like 1/4 power. That way you get the same output every time.

I haven't photographed frogs at night, but have photographed people in pitch black. Use the illumination from the IR on the flash to make sure you have the right area, use the center focus point (crop later), and fire away. Let the flash work for you.

Hope some of that helped.