I dont have any expirence with the H2 specifically, but I have setup a handful of amps before. Most wiring kits will have a fuse on your 12V line, as long as you remove that fuse you should be fine without disconnecting the battery. If your 12V line doesn't have a fuse you can still run it without disconnecting the battery, you just have to be more careful(touching metal with a live line can give you a nice spark show
).
As far as the ground...most any metal surface should suffice. I just have mine screwed into the body in my car and in my truck, and both work fine (used factory screws under the carpeting). I wouldnt tie it into the stereo ground, but that is probably just me being paranoid (I can live without my amp, but if I fried the stereo I would be pissed).