CipSoft only stated that monster mitigation has been increased, without disclosing further information. However, comparing some old values with the new ones below, it is possible to notice that the increase is not a fixed value, and varies between 55.56-60%:
Bat (0.15 - 0.24%): +60.00%
Deer (0.05 - 0.08%): +60.00%
Goblin (0.20 - 0.32%): +60.00%
Butterfly (0.36 - 0.56%): +55.56%
Cat (0.05 - 0.08%): +60.00%
Cream Blob (0.36 - 0.56%): +55.56%
Blood Beast (1.18 - 1.87%): +58.47%
Ancient Scarab (0.86 - 1.35%): +56.98%
Green Djinn (0.78 - 1.22%): +56.41%
Crazed Summer Rearguard (2.11 - 3.36%): +59.24%
Dark Torturer (1.74 - 2.76%): +58.62%
Hellflayer (1.60 - 2.54%): +58.75%
Vexclaw (1.91 - 3.03%): +58.64%
Demon (1.74 - 2.76%): +58.62%