I had the same problem with a much bigger wall and I was using a small sledge hammer. No matter how hard I swung the hammer the wall wasn't moving. So I got a bigger sledge hammer . . . one that has a 4 foot handle and a 12 pound head. It was about $20 at Harbor Freight (cheap tools but they work for weekend jobs). I took one swing at the top (it was actually a concrete counter top on top of a big wall) and it broke into 5 big pieces that I could then break down. It was amazing. Crazy what a difference a bigger hammer will make.
See if you can get hold of a big heavy hammer. You will most likely need a masonry chisel too, not least as if the mortar is hard it will be quite tricky to remove from underneath the bottom course of bricks.
This is the kind of "occasional" job where it's handy to know someone (friend? family) who you can lend you the tools for the weekend. Or, hire them.
I've been doing a similar job. I started with a 7lb sledge hammer, which either didn't work (for the bricks that are backed into a solid earth bank) or, in the case of freestanding brick wall sections, worked so well that it would send dangerous shrapnel flying at a glass window! So I bought the cheapest SDS drill I could find (£40 - thank you Wickes) and a 40mm-wide chisel bit, and went at it that way, raking the mortar out, then holding the bit to vibrate the bricks out. It's working pretty well, but it's hard work and slow.