Ive RBTA,
I leave them on the rock when I cut them,and I put the rock back where it was when Im done cutting.That stuff is to help with stress.
I just use a cheap retractable razor knife.(the ones with the long blade)
First I put the rock out of the water,then find the mouth on the anemone by flatting it out,then I cut it in half!Then return the rock.
Even if you cant get on all the base because its in the crevice of the rock.its fine, It will finsh the spit on its own.
It dont really get messy.sounds messier than it is.might want to run some carbon?