The discovery of ambient-pressure nickelate high-temperature superconductivity provides a new platform for further exploring the underlying superconducting mechanisms. However, the thermodynamic metastability of Ruddlesden-Popper nickelates Ln_n+1Ni_nO_3n+1 (Ln = lanthanide) poses significant challenges for precise control over their structures and oxygen stoichiometry. This study establishes a systematic approach to growing phase-pure, high-quality Ln₃Ni₂O₇ thin films on LaAlO₃ and SrLaAlO₄ substrates by using gigantic-oxidative atomic-layer-by-layer epitaxy. The films grown under an ultrastrong oxidizing ozone atmosphere are superconducting without further post-annealing. Specifically, the optimal Ln₃Ni₂O₇/SrLaAlO₄ superconducting film exhibits an onset transition temperature (T_c,onset) of 50 K. Four critical factors governing the crystalline quality and superconducting properties of Ln₃Ni₂O₇ films are identified as follows. 1) Precise cation stoichiometric control suppresses secondary phase formation. In an Ni-rich sample (+7%), the thin film forms an Ln₄Ni₃O₁₀ secondary phase, and the R-T curve correspondingly exhibits metallic behavior. In contrast, an Ni-deficient sample forms an Ln₂NiO₄ secondary phase, with its R-T curve indicating insulating behavior over the entire temperature range. 2) Complete atomic layer-by-layer coverage minimizes stacking faults. Deviation from ideal monolayer coverage induces in-plane atomic number mismatch, which directly triggers out-of-plane lattice collapse or uplift near bulk-equilibrium positions. 3) Optimized interface reconstruction can improve the atomic arrangement at the interface. This can be achieved through methods such as annealing the SrLaAlO₄ substrate or pre-depositing a 0.5-unit-cell-thick Ln₂NiO₄-phase buffer layer, which enhances the energy difference between the Ln-site and Ni-site layers to promote proper stacking. 4) Accurate oxygen content regulation is essential for achieving a single superconducting transition and high T_c,onset. Although the under-oxidized sample demonstrates a relatively high T_c,onset (50 K), it displays a two-step superconducting transition. Conversely, the over-oxidized sample exhibits a reduced T_c,onset of 37 K and similarly manifests a two-step transition. These findings provide valuable insights into the layer-by-layer epitaxy growth of diverse oxide high-temperature superconducting films.