I have used several slide hammers like it, and they do work, however you are going to still have to beat it.... If having problems with a 60# demo/jack hammer, then you have problems it may not solve other than providing another option to do the work. Which when ramping up the tool to the job is not a bad idea.
Either way it sounds rough, a story I know well. Here its a roll of the dice if you're gonna have serpantine bedrock or sand. Sand it will install itself, rock you can beat all day long unless you find a crack. The trick is finding the crack.... And still chance a 1/2' per hour.
On those days when i have to do the pounding, or when nice enough to lend a hand to the lower wage slaves who I have tend this, I often day dream of something like this drilling rig. set up and let rip. http://www.portadrillmini.com/index.php?a=pdmMain
If you know of a freindly fab shop, or own a welder, you may be able to make a lighter duty version to mount a long core drill or extended length hammerdrill to. As for bits, I do know of several siezmic shops that have made thier own for purposes like this, to drive pins and grout. Simular to what this guy makes... http://www.hydratek2000.com/
Sounds like the type of soil your in would be like cementing it in afterwards....