I found it quite easy if you have an angle grinder and power file. Whatever you use don't be tempted to cut too much off at once-at the most one coil at a time. Flatten the cut end with the electric file/angle grinder, replace and test.
Trying to cut a tempered steel spring with a saw or hand cutting tools is quite an effort-you really do need power tools not just to reduce the effort but also to make a really neat job.
Good luck.