I'm planning to build a cabin on poles, and build it into a hillside.
At the top of the hill the posts (6x6 treated), will be about 2 feet off
the ground. At the bottom of the hill they will be about 9 feet off the
ground. I'm intentionally building this way to achieve a unique look,
and want to build a deck over the high side. Everything is planned,
except I am not sure how to get the posts in the ground so that the
layout is square. Normally when building a pole structure, you use a
string and make sure that the "X" between the opposite corners is the
exact same length. (in other words, run string from front left to rear
right, and vice versa. When the lengths are the same, it's square.

However, I'm dealing with a hill. Running the string up and down the
hill is going to change the lengths. This leaves me totally puzzled how
to get the posts square. Granted, I could put the posts in the ground
and measure from the tops of the posts (which would be level), but I
sure dont want to have to keep re-drilling the holes in the ground until
I get it right, not to mention handling heavy poles over and over, and
trying to keep them upright without any fill around them. There must be
a better way? Does anyone have any idea how to do this? Will a laser
level help in any way (other than getting the tops of the posts level)?

I've done a lot of building over the years and have built several pole
buildings on flat ground, but this project has me puzzled.

Any experts on this sort of thing out there?

Thanks