(1) Background: The growing demand for developing new methods of degraded land remediation is linked to the need to improve the soil environment, including post-industrial soils. Biological methods such as the aided phytostabilisation technique are the most common methods applied to achieve effective remediation. This study aimed to determine the technical potential of methods using novel or yet not used soil amendments, such as blast furnace slag (BFS) and coal slag (CS), with Dactylis glomerata L. as a test plant. (2) Methods: The experiment was conducted on post-industrial area soil with high concentrations of Cu (761 mg/kg), Cd (23.9 mg/kg), Pb (13,539 mg/kg) and Zn (8683 mg/kg). The heavy metal content in roots and the above-ground parts of plants and soil was determined by flame atomic absorption spectrometry. (3) Results: The addition of BFS to the soil was the most effective in increasing Dactylis glomerata L. biomass yield. The Cu, Cd, Pb, and Zn concentrations were higher in the roots than in the above-ground parts of the plants. BFS and CS induced a considerable increase in soil pH, compared to the control treatment. The addition of BFS also produced the greatest significant decrease in the Pb content in soil following the phytostabilisation process. (4) Conclusions: In view of the above, the use of BFS in the aided phytostabilisation in soils contaminated with high levels of Cu, Cd, Pb, and Zn can be recommended for larger-scale in situ projects.