ximena, the normal method of removal for this is to begin with a hammer, no chisel. Hammer the concrete working in a line, back and forth, across the floor; this will begin to give you a first breaking point. Then work to divide this piece in half using the same method. Once you're able to pull out a few peices the rest will break and come out easily.
Before tiling this, you'll need to replace this concrete bed and let it cure thoroughly. I'd wait 2 days before tiling, and 3 more before grout.
And yes, 1/4" per foot drainage is the minimum required, so be sure to maintain at least that.