Hammer drill holes in it, every couple of inches. Bigger holes are better. Clean and fill with water the evening before a hard freeze, apply good sized sledge hammer to the remnants after things warm up.
I've always been a curmudgeon - now I'm an old curmudgeon. ~Molɔ̀ːn Labé Skýla~
How big is big? I’ve moved all kinds of rock…. Blasted granite for highways…. Haul trucks, scrapers , D11’s, you name it. If you only need to take the top off to get it below grade I’d image it wouldn’t be too bad with a hoe ram on an excavator or even a backhoe…. Just depends how big it really is. I’ve seen the guys use that grout stuff but if your gonna have to drill a bunch or holes then might as well just use a hoe ram.
I have removed large concrete pillars using Japanese dynamite. Drill holes, at a slight angle down, fill with the product. Come back the next day, remove what broke off and repeat. Great for inside demolition where heavy equipment can go. It will break the rock, dig down as far as you want the rock under the drive, drill horizontal fill holes and wait.
The stuff is nothing to get careless with, wear protective clothing and face mask... you don't want to have a blowout while pouring or mixing.
That being said you can purchase Dexpan at just about any Home Depot and the 1-1/2" carbide tipped star drill. One Eleven-pound bucket $55 should be enough for between 18 to 25 holes spaced in a grid of a foot apart. And you can probably rent the drill and rotary hammer right there. The drill cost about $50 to purchase. I'd mix just enough to do a few holes first.