Infinite Regeneration

Infinite Regeneration (無限再生) is an Intrinsic Skill that has been granted to Rimuru Tempest as Demon Slime by the Voice of the World during his Harvest Festival ascension to a True Demon Lord.

Abilities
Infinite Regeneration is vastly superior to the Ultraspeed Regeneration. While Ultraspeed Regeneration becomes disabled by Spiritrons, Infinite Regeneration continues working under those conditions by increasing the Magicules input.

That means Rimuru would be able to withstand at least one Melt Slash or the like while standing still.