Cut plasterboard out neatly in the location of the brackets on the back of the unit, fix 3x2 noggins between studs to the back of the plasterboard, then fit the square you just cut out back. Or use a cabinet fixing rail from Ikea and ensure its fixed to the studs as well as the plasterboard. Use redidrivas for plasterboard fixing.
Thank you for replying Jord86. However I don't want to do all that. I was thinking of using GripIt fixings or Spring Toggles.
use additional fixings into the studs behind. For example use 50mm L brackets above and below the wall unit securely screwed into the studs and into the top / bottom of the unit. if the back is thick enough you can also screw straight through into studs for additional strength.