The localizer (lateral deviation indication) is going to be harder to spoof due to the beat/squeal that it would introduce when broadcast alongside the legitimate localizer. The pilot (or PNF) will monitor the audio signal for the Morse code that identifies the localizer and the squeal will be apparent.
The glideslope signal (vertical deviation indication) would be easier to spoof. Set that up 1/2 mile short of the runway and aligned to intercept the proper glideslope shortly before the true glideslope intercept point (Maltese cross on the chart). That has a chance of working and going undetected. If you're able to get an aircraft onto the rogue glideslope lobe, even when ATC gets a low altitude alert, the crew is likely to report they're perfectly on glideslope. I'm not sure this is as practical an attack as simply firing on an aircraft on approach, of course.
The glideslope signal (vertical deviation indication) would be easier to spoof. Set that up 1/2 mile short of the runway and aligned to intercept the proper glideslope shortly before the true glideslope intercept point (Maltese cross on the chart). That has a chance of working and going undetected. If you're able to get an aircraft onto the rogue glideslope lobe, even when ATC gets a low altitude alert, the crew is likely to report they're perfectly on glideslope. I'm not sure this is as practical an attack as simply firing on an aircraft on approach, of course.