For tall hedges, there are two solutions.
First method is to get an arborist ladder, these are the most stable for working on trees or tall hedges and
should be fairly easy to find by searching the net.
Second method is to purchase a pole saw, these can be had either as a manual saw or with a chainsaw attached, the chain saw models come both in electric and gas powered but they are normally not longer than 12-15 feet in length.
The ladder method would allow you to use loppers, hand pruning saws and other tools ( electric hedge trimmers, chain saws, pruning shears ) as well as put yourself in the best position to accomplish the work.
Arborist ladders come in two designs a tripod design and a wide base design that looks like a splayed standard step ladder. I prefer the tripod design for tree and hedge work since it allows me to get the proper, safest position.
Regardless of the method you use, the correct shape for tall hedges is wide at the base narrowing to the top, this is so the sun isn't shaded anywhere which can result in bare spots that just look ugly in a hedge.
If you are trying to maintain an established shape, build some forms that you can follow to check that you have not ruined the preset shapes.