Just because you have a tough body and well kept armour doesn’t mean you are invincible from all but the strongest attacks. As any member of the Imperium knows, if you have enough lasguns, the chaos beast will eventually die.
If an character (be it player or npc) is to take a number of hits in a round greater than their toughness bonus, whether they damage the character or not, they make a Toughness test. Failure results in the character taking a single wound. If they have taken more hits than twice their Toughness Bonus, regardless of the damage dealt, they must make a Toughness test, failure resulting in the character taking a level of fatigue, in addition to taking a wound.