As the report says: the aircraft was then about 350 mtrs from the runway at an altitude of 250-300 ft (80-100 mtrs). In that case even a LMG or MMG with AP rounds would do the trick. And both of those weapons are man-portable, by a single man. While the HMG with tripod would require 2-3 men, not difficult.
The attackers were waiting in the "approach funnel" to the runway. With an approaching target from dead-ahead, it was quite easy.
Of course all that begs the question: was there any perimeter security in place at all? Especially considering that it is a PAF Base and not a small one at that.