Fergus' confidence came from genuine battle experience and tests. The man had every right to boast about his defensive prowess, and Khan had long since acknowledged his power.
However, every technique had flaws. Fergus' spell used his body as the foundation, which had weaknesses to begin with. Reinforcing and altering skin and muscles could hide and protect those critical points, but they didn't disappear.
The joints, the tendons, the various extremities, the spine, and other sensitive spots still existed under that shield of skin and muscles. The eyes were even more exposed, and Khan didn't hesitate to go for them.
Fergus obviously knew his weaknesses, too, but nothing could have prepared him for Khan's offensive. His list of abilities was long, but he still kept secrets, and the latest attack had brought an ominous revelation. Khan was a living weapon capable of turning mere air into a deadly instrument.