Have you thought about a demo saw, a recirculating saw, a sawzall? A 2-3 lb hammer? Pry bar, crow bar, various digging bars?

Any pictures?

The mortar bed under tile is usually somewhat weak. I'd start with a cold chisel and heavy hammer and see if it breaks into pieces you can deal with. Maybe start with an edge.

For that thickness I would use a masonry chisel or a 3/4" wide cold chisel (similar to this one https://www.homedepot.com/p/Dasco-Pro-3-4-in-x-7-1-8-in-Cold-Chisel-408-0/100165999) and a 2-3 lb hammer (similar to this one https://www.homedepot.com/p/DEWALT-2-lb-Fiberglass-Drilling-Hammer-DWHT56141/308469803) . Just start near one edge and break the mortar off in chunks and strips. You can also try splitting it more toward the middle and it might start coming apart in big chunks. I've used this procedure to break up masonry under a fireplace and it came apart quite quickly.

Bruce