Zincy
In Vino Veritas
The emperors personal guard, the companions (at the time numbering about 10k), held off the entire warp for 3 years in the War for the Webway (they had some Mecanicum and Sister of Silence help, but they did most of the fighting). They only started taking any real losses in the final year when Terra basically phased into the Warp partially, and the traitor legions and traitor Mecanicum showed up.
Even then, they effectively held off the forces of hell until the fall of Horus and Big E becoming the world’s angriest lighthouse.