This ability only exists in Stage 2. In Legacy Evolve, Assaults have a similar ability but it acts as a shield instead of a damage reduction.
The Defense Matrix is the Class Ability of the Assault Class. The Defense Matrix creates a metallic shield around the Assault, and greatly reduces damage taken by 55%, allowing the Assault to take a brutal beating and lose little health in the process.
The Defense Matrix lasts for 19 seconds and has a recharge of 30 seconds. All Assaults have the same Defense Matrix, with the exception of Renegade Abe, who has a slightly weaker Defense Matrix. His Defense Matrix only protects him from 45% of damage dealt, but he makes up for it with his ability to self-heal.
During the beginning of every match besides Arena, a Defense Matrix will be granted to every Hunter for eleven seconds. This is to discourage Monsters from attacking Hunters at the very beginning of the match.