How can I remove concrete from concrete?

[BradN]

Hi guys. This is the current sorry state of our narrow side yard. We've removed the old paling fence and our old raised garden beds and are going to drop in new metal garden beds like the one my partner and project boss has assembled in the pic above.

The problem is that the new beds aren't going to sit level because of the concrete that dripped down along the fenceline when our neighbour built the retaining wall and core-filled the Besser-block fence pillars.   

What would be the easiest way of removing some of that concrete from the bottom of the retaining wall so the raised beds can sit flat? It won't crack off easily just by hitting it with a pick. Would an angle grinder be suitable? Terrible pic below of the concrete or mortar at the base of the retaining wall:

Also, would it be a good idea to paint the retaining wall and fence pillars? If so, what sort of paint would we need?

When it rains some water seeps or dribbles through the retaining wall in places where it can find gaps in the mortar, and we found a fair bit of green algae growing around those spots when we recently took down the old paling fence. It's not a big problem to clean that off, but the whole bare wall looks pretty ugly... 

One last question: It seems that corrosion has narrowed the holes in this 50mm metal tubing so that I can't fit 3mm clothesline through it any more. Could I slightly widen the holes with an 18V drill? What sort of bit would I need? I don't know what kind of metal it is.

Anyway, thanks in advance for all your sage advice. Here's a bonus pic of our silly old dog Night catching some sun by the old timber retaining wall. He's a Very Good Boy.

Cheers,

Brad

[BradN]

Thanks @EricL . I was surprised what a difference it made. It looks so much better than the bare wall, even if we'd painted the wall instead. And yes, I think we'll be getting more panels to extend things in both directions, but not right down the side of the house where the garden beds are. There's a gate that separates that little side yard from the back yard.

I was a bit apprehensive because I hadn't drilled into masonry before (and still haven't -- Sally did that here). I was worried that we'd mess something up and the anchors would fall straight out of the wall, or that the battens wouldn't sit flush enough or something, but it all went very smoothly. Just goes to show that you never really know what you can do until you try it!

Just as well that we had the brick bolster -- we needed it to chisel away some mortar or concrete where the battens were going. It's earning its keep as a pallet chisel too.

Cheers,

Brad