As a classic high-pressure superconductor, elemental lanthanum (La) offers a vital platform for understanding tunable superconductivity. Using first-principles calculations, this study systematically investigates the strain-engineered superconducting properties and the underlying multiscale mechanisms of the ambient-pressure La-$P{6}_3/mmc$ phase. We reveal that uniaxial compressive deformation uniquely induces an anomalous enhancement in the superconducting transition temperature $\left(T_{\mathrm{c}}\right)$, which peaks at 7.5 K (a $\sim 20\%$ increase from the pristine 6.2 K) at an optimal strain of $\varepsilon =0.048$. Remarkably, this optimal superconductivity emerges despite a monotonic decrease in the global density of states at the Fermi level. We demonstrate that this paradox is resolved by a strain-driven $5s\to 5d$ interlayer charge transfer, which establishes highly localized flat-band pockets and pristine Fermi surface nesting near the $k$ point. This momentum-specific nesting sharply amplifies the electron-phonon coupling matrix elements and induces selective low-frequency acoustic phonon softening. However, excessive compression triggers a closed-to-open Lifshitz transition that destroys the nesting condition, abruptly suppressing $T_{\mathrm{c}}$. Conversely, biaxial tensile deformation monotonically reduces $T_{\mathrm{c}}$ due to a severe energetic misalignment, where localized electronic states are pushed deep below the Fermi level, rendering them ineffective for pairing. These results reveal the key interactions between orbital occupation redistribution, topological transition, Fermi surface nesting, and lattice dynamics, providing a microscopic insight into the strain-tunable superconductivity in elemental La.

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• Strain
• Superconductivity
• Elemental metals
• Density functional calculations