Agree except NO heat gun... use steam and a putty knife. A heat gun used on mastic and/or unknown "varnish" can (and will) cause toxic fumes...

Source: I have refinished a third of the wood floors in my 122 yr old house, most of which have/had at least 3 layers of linoleum and 1 or 2 layers of carpet and foam padding... ended up hospitalized for acute respiratory distress due to chemical pneumonia from using a heat gun to soften the mastic... Took 5 months to recover fully.