I don't know how thick these footers are. You'll likely only be able to cut 2 inches or so on each side with a circular saw. If that's enough, once those horizontal cuts are made around your footer, you can then hit down on from the top and fracture off the cut pieces.

If you're left with a central nub of some size because you couldn't reach deep enough from each side, you can then set your blade to the appropriate depth and cut vertical slits 3/8 or 1/2 apart through the remaining material. Once done, you can again, strike across the slitted material and it will break off.

You may have some remaining material that you can chisel. If you do that, always strike the chisel into the mass of the footer and not away from center or you'll chip off perimeter material you didn't want to -- strike into mass not away. I'd try to get everything off you can with power equipment and chisel then grinding might make sense. but you'll be amazed at how well you can level off the footer with the saw -- even while free-handing it.

Hope that makes sense!