Removing tiles from a wall can be tricky, even if you're taking all of them off and not trying to salvage them. The problem is that walls are more fragile than floors, so the floor-tile approach of breaking up the tiles with a hammer won't work, as it will damage the wall behind them. Instead, you have to take the tiles off from the sides by breaking the mortar bond between tile and wall. After the first tile is out, it becomes easier, because you can work at the sides of the other tiles from there.
Extract the grout from the perimeter of one of the tiles by scraping your grout saw over the grout and digging down into it. Get as much of it out from all around the tile as you can. If there is a loose tile, start with that one; if not, start anywhere.
Set the tip of the mortar chisel against the side of the tile (in the line exposed by the grout extraction) with the chisel at a shallow angle where the tile meets the wall. Tap the chisel with the hammer. Move the chisel to another part of the tile and repeat. Continue working your way around the tile with the hammer and chisel, tapping rather than prying, until it comes out.
Remove the next tile by chiselling at the exposed side of it and working from the open space created by the extraction of the first tile. Concentrate on breaking the bond between the tile and the wall by chiselling at a shallow angle rather than trying to pry it out. Continue until the tile comes out.
Repeat for each tile, getting each one out by chiselling from the side where you've already removed a tile. Remove all the tiles.
Scrape the wall with a putty knife to remove any residual mortar. Go over the whole surface with a belt sander to get it smooth and flat. The wall is now ready for re-tiling or other resurfacing.
Wear eye protection when chiselling tile.