An armored man can be defeated in many ways.
Penetration. A sufficiently powerful blow can penetrate the armor far enough to injure the man inside. This is most likely to happen where the armor is thin. A pollaxe spike can penetrate very heavy armor, but it is a heavy, relatively slow weapon and the blow must be delivered with great precision to hit properly without glancing.
Impact. A heavy blow can do damage without penetrating the armor. For example, a sufficiently heavy blow to the head can stun a man inside his helmet, a smashing blow can break his fingers inside his gauntlet, and an edge blow against mail can bruise without penetrating or even break bones where they are close to the surface. A violent throw can injure a man inside his armor when he hits the ground.
Intrusive. Some attacks avoid the armor entirely: Targets include the eyeslots or open face, palms, behind the gauntlet cuff, back of the knees, stabbing upward from beneath a mail skirt, and sliding up under an aventail. Thrusts with acutely pointed weapons can also be intrusive, penetrating the center of the mail rings far enough to draw blood without actually breaking the metal.
Dislocation. Some wrestling techniques can dislocate or break limbs inside armor, and weapons can be used as levers to achieve the same result.
Often an attack will not be intended to cause instantaneous injury, but to set the opponent up for a following attack. These include:
Uncovering. Blows can be aimed to drive up a visor so the face is exposed, or cut or break the laces holding up shoulder armor so that it slides down to expose the shoulder and hamper the wearer.
Disarming. Many techniques were used to either disarm an opponent entirely, or to beat a two-handed weapon out of one hand so it was temporarily ineffective.
Binds. Grips and locks can be used to control an opponent's arm or weapon so the he can't defend or attack effectively, move him into a vulnerable position, or prepare him for a throw.
Throws. Throws and knockdowns can not only cause direct injury: an opponent driven to the ground is much more vulnerable and less effective.
Exhaustion. The exertion of attack and defense in armor can render a man helpless with exhaustion or even kill him with heat stroke. In the Katrington-Annesley duel of 1380, Annesley was nearly blinded by sweat and close to exhaustion and Katrington died of heat stroke.
Am I missing any important category?