Matter condensation

The Master uses his Tissue Compression Eliminator. (COMIC: The Heralds of Destruction)

Matter condensation was a miniaturisation technique used by the Master to kill a living being by shrinking it.

According to the Fourth Doctor, his nemesis had picked up the technique "somewhere on his travels", and this way was a "nasty sort of death". (TV: The Deadly Assassin)

The Master employed a Tissue Compression Eliminator to achieve the process. He used the weapon frequently over the years. (TV: Terror of the Autons et al.)