Question: Use of Assault Artillery (or Tank Guns in general) against Infantry

[Imp]

World war II era vs dug in units roll it up as close as is reasonably safe.
Use mainly just the MG till accuracy increases by using W key or switching off the main weapon. Once accuracy is up fire everything.
If possible use them against grouped units to spread suppression even if it misses. Used this way you will get the odd solid hit but main thing I use it for is to spread enough suppression that combined with other units the defenders leave their fortification.