Did you skip the part of the text about the vickers (or bofors) in front of the mortar pit because you think they would stop you pretty quick along with any other infantry in the area and even if you get into the building its gotta take a lot of time to burn it down with its brace.
To counter combined arms, one must use combined arms.
If there is a building with a vickers in it and a mortar pit behind it, you could consider tactics such as dropping smoke using a mortar to nullify the vickers while you close in with flamer pioneers.