Which is better/safer for cutting concrete slab, hammer drill or angle grinder?
The standard approach used by electricians routing large conduits through concrete floors is using a diamond edged hole saw cutting 3 or 4 inch holes. Several of these holes overlapping could do it. Or several holes near each other might suit your purpose. These drills need to be water cooled while drilling. You may be able to rent one.

Alternatively a smaller grinder could score the cutout on both sides, a drill could then break through the webbing left under the scoring and a chisel to finish it off.