I don't get it, how did they stealthily march a force large enough to just roll over everyone? They even bring a huge carriage filled with sofa and adorned by multiple flags, are they even trying to hide?
"Don't use magic artillery, just attack everywhere" sounds like that joke about how the Nazi shouldn't have wasted time siege Stalingrad and instead just take the city, it doesn't even make sense.
From the looks of things, it's taking four things into account.
First, the previous chapter showed an enemy unit moving over rather rough terrain under the cover of darkness, suggesting that they were either slowly infiltrating No-Man's-Land in preparation for this assault or were moving into a weaker section of the defensive line because the terrain they had moved through was considered either impassable or improbable.
Second would be the fact that they're using pseudo Stormtrooper tactics. A "smaller" force (compared to a general attack, "small") assaulting a less defended or weaker portion of the enemy defensive line in order to force a breach, followed by the general infantry to exploit it. The many uses of grenades during the assault sort of suggests this, though there should have been more use of pistols and melee weaponry if so.
Which then leads to the third point. The tech level of the story is somewhere between Great War and Early Interwar (1916 to 1925). Meaning that, while radios are around they're mostly in use with the Navy and ground troops generally made due with telegraph at best, couriers at worst (think the movie 1917). Just even receiving news from commanders at the front is going to take half an hour to an hour, and this isn't even considering conflicting information from different line units. Which leads ro point four...
The strategy looked like it called for a general assault right after a breach was forced in order to mask the fact that there was, in fact, a breach of the first trenchline. Combat is very, VERY chaotic. Incomplete and conflicting information reaching command is almost a certainty with the previous lag time in communications I mentioned, and the unit at the breach is kinda dying a bit too quickly to actually get the word out. This could have been mitigated by things like signal flares, but since we didn't see anything of the like mentioned, the signalmen were either killed too quickly or they just never had them.