— Official game quote
The FlaK 38 is a 20mm anti-aircraft gun used by the Germans. It is very dangerous early in the game (just as a PAK-38), because of its infantry-shredding 20mm flak shells and rather fast firing, which can annihilate even structures that lack proper reinforcement.
The Flak is very dangerous to Cam Nets and units underneath them, and can shred any unit left in the open, a very good reason to try and draw fire towards your bunker. The best way for dealing with this is to shoot the crew off or use AT weapons to blow it up, it also leaves no wreckage.
It will move up to its position, then settle down and unleash hell.
Flak 38's are very common in the early waves, and to see one before wave 30 isn't rare (although it would be painful).
Having a good sniper w some xp and a skilled scout would help