There are a number of different plasterboard fixing on the market but being honest I wouldn't trust any of them with 12KG. When we fix heavy things on a plasterboard wall we never use plasterboard fixings. The best way is to work out where the fittings go. Cut a slit into the plasterboard next to the position of the fittings, behind where the mirror goes if possible so it's not seen when the mirror is fixed. Then thread a baton as long as you can get it in behind the plasterboard and screw through the plasterboard to hold it in position. If you hit a stud you may have to put a baton in either side. Put back in position the piece of plasterboard you cut out, then fill any gaps or holes and sand back if required. You can now screw your mirror fittings through the plasterboard and into the baton. The baton will distribute the weight. It sounds more difficult than it it is but is very effective.
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