The Fixed Weapon class includes a variety of devices from anti-infantry machine guns to full-sized artillery weapons. Typical weak spots are the gunner for anti-tank guns/heavy machineguns and the slit/rear for bunkers.
Valkyria Chronicles 2
Bunkers lose the Slit weak spot, but gain a radiator above the rear door. As with vehicles, though, standard bullets won't do much damage to the radiator. Manned guns are replaced by turrets, which are harder to kill as they will turn their armoured front to face approaching units. They are weak from the unarmored sides and rear, which even standard bullets will rip apart.
A new addition is the Cannon Tower, which is a tall tower with a rotating turret on top, possessing long range, high defense and extremely heavy firepower. Its weak point is a radiator on the rear of the turret, but sneaking behind one and shelling/lancing its radiator is quite difficult, since it tends to notice approaching units at an extended distance and will turn around to intercept. The easiest way to kill these is with flamethrowers. Flanking AT Sniper shots to the radiator also work but will take longer. Lavinia with a sufficiently powerful AT turret can brute-force Cannon Towers from the front.
AT sniper rifles tend to do more damage to fixed weapons than they do to vehicles.
Valkyria Chronicles 3
Fixed weapons here are very similar to those in VC2. The differences are that radiators are more vulnerable to standard bullets, turrets are stronger, and Cannon Towers have less range but deal more damage, the latter due to attacking from above.