Depends on what they're mounted on. The goal is to always 1) limit the amount of tissue you're cutting and 2) leaving enough of a base so that the glue doesn't come in contact with the tissue.
If it's on a plug your best bet is to score the tissue with a scalpel or razor blade and then use bone cutter to break the plug. The only issue here is that you're 1) dependent on the plug being a breakable type and 2) that you get a favorable break. You can take the bone cutters right through the tissue but that's a sloppy way of doing it.
Alternatively you can run it through a saw. The band is pretty narrow but you might still end up sacrificing some polyps. The draw back here is that, while extraordinary useful, you're looking at $400-500 for the saw. I would advise against a dremel tool. A long long time ago I used one and found 1) it wasn't precise, 2) generated a lot of heat, 3) even with specific blades it didn't always cut the best 4) very messy (good way to grind up zoas and aspirate it).
Your other option is the 'scrape and wait' where you can use a scalpel or exacto knife to remove the polyp(s) from the plug. You can then drop the polyps into a shroom box with a thin layer of coarse substrate and give it a week or two to heal and attach. You can then mount these on plugs.
Which ever approach you take you should always give all frags and broodstock a 5-6 min lugol's bath before mounting. You also want to change that water out every so often if you're fragging a lot of stuff so that you're not putting stressed animals in a bath with juice from other stressed animals. Finally, remember that all zoas have different tolerances for fragging.
