Structure formation is thought to act via hierarchical mergers and accretion of smaller systems driven by gravity, with dark matter dominating the gravitational field. Combining X-ray and optical imaging and spectroscopy provides a powerful approach to the study of the cluster dynamics and mass assembly history. The ROSAT–ESO Flux Limited X-ray (REFLEX) distant X-ray luminous (DXL) sample contains the most X-ray luminous galaxy clusters (LX≥ 1045 erg s−1) from the REFLEX survey at z = 0.27–0.31. We present the photometric (Wide Field Imager) and spectroscopic (VIsible Multi-Object Spectrograph) data for the DXL cluster RXC J1131.9−1955 (Abell 1300); in combination with the existing X-ray data, we determine and characterize the substructure of this post-merging system. We analyse X-ray-selected groups in a 30 × 30 arcmin2 region encompassing the cluster in order to study the mass assembly of A1300. The X-ray surface brightness map of A1300 appears disturbed and exhibits the signature of a forward shock, which is consistent with a previous analysis of radio data. Moreover, we detect a large-scale filament in which the cluster is embedded and several infalling groups. Comparison of the whirlpool-like features in the entropy pseudo-map of the intracluster medium (ICM) with the distribution of the cluster members reveals a direct correspondence between the ICM structure and the galaxy distribution. Moreover, comparison with existing simulations allows us to better understand the dynamics of the cluster progenitors and to age date their impact. A1300 is a complex massive system in which a major merging occurred about 3 Gyr ago and additional minor merging events happen at different times via filaments, which will lead to an increase of the cluster mass of up to 60 per cent in the next Gyr.