Trimming question ...

I made a mistake buying the Echo HC-150 20-inch hedge trimmer:
http://www2.picturepush.com/photo/a/...g/12891035.jpg

The trimmer is (way) too small to make the tops flat of my
Oleander bush.

I can't get the 20-inch blade across the top, while standing on
a step ladder, from the one side. The problem is that the other
side is wholly inaccessible to a ladder, so I can only trim the
top of the bush from one side.
http://www1.picturepush.com/photo/a/...g/12891079.jpg

The Oleander bush is something like five or six feet across,
and something like nine or ten feet tall, for something like
a few hundred feet along a cliff-like extremely steep hill.

I can easily climb up on a step ladder to cut the side, but,
I can only lean over about three or four feet across the top
with the puny 20-inch 21.2cc hedge trimmer - and even that is
risky because one fall could be dangerous with a hedge trimmer
(I've already sliced my thigh with the thing in the past).

So, I'm mostly asking if there are better ideas for how to
cut the top of a tall and thick Oleander bush when you can only
access it from one side, and when your trimmer is 20 inches
while the bush width is at least three times that.
http://www2.picturepush.com/photo/a/...g/12891105.jpg

Any helpful suggestions?