Hello

The rear garden (never turfed) of a new build (2 years old) is uneven, on one side - I would like to place a retaining wall to level the remainder of the soil. I began digging a 300mm deep by 400mm x10m trench.
I plan to do another trench running at a right angle for a summer house, and another in parallel, think of it as a U-shape although the parallel wall is not retaining so the foundations can be 150mm deep which makes it slightly easier.

The question I have is whilst I was digging the topsoil (dug up like clay) - I came across a much harder level with many stones. I was not able to dig this, just chip at it (I have little gardening experience). I wondered if this could be compacted hardcore or if there are any tips to break it down. I think a micro digger may be difficult to get into the rear garden due to a very narrow side passageway, plus a bit expensive hire wise for what I hope to achieve.

What tool or power tool should I be using to make this a little easier?
Has anyone any tips on dealing with hard/stone ground? (This is a mix of large stone and small stones) Or is it just a case of enduring blisters and keeping at it.

Thanks
Chris