Sharp, fine tooth hardpoint saw does the job well. Given how cheap the things are from the usual DIY suppliers its well worth buying new if yours has been around for a while. Fine tooth gives better control on breakthrough but the coarse tooth one does well enough if you support both sides..
Best tool I've seen for trimming down along a length is the 18 V battery Makita circular saw with the thinnest blade. Builder who helped me with a job last year had one and it did a super job. He really rates the thin blade but said it doesn't fit a normal circular saw. Having seen that one in action I'd buy one tomorrow despite the price if I didn't already have a decent Hitachi mains powered one that will almost certainly see me out. Don't advise trying a normal circular saw or even the small plunge saws. Can be done but its easy to end up with a spectacular mess. I did get good results with my small plunge saw after "some practice" on the stash of offcuts left from the main extension job.
The half circle blade in a multi-tool does a decent job but its slow and the swarf very fine making it four times as as pestiferous as the usual stuff.
Clive.