Deployment. Artillery at the back, infantry in a line you intend to hold, and your designators placed with the 12 inch geometry already in mind. Use Eager Advance's Scouts move to set your forward edge deliberately rather than accepting where deployment put you.
Round one, take Creeping Barrage. Almost everything the enemy owns is more than 12 inches away on turn one, so this is when the bombardment hits the most units. Shaking three units at Strike Force, each losing 2 inches of Move and 2 on Charge rolls, meaningfully delays the whole enemy advance and buys you an extra shooting turn.
Designate before you shoot, every turn. Signal Flares happens in your Shooting phase, so mark the unit you intend to kill, then fire. Against a target sitting in cover, switch the bombardment to Incendiary Bombardment so it cannot have the Benefit of Cover, and shoot it with everything.
Protect the designators. They are the reason the list works and they are cheap enough to be shot off casually. Sump-Smog Screen (1CP) gives the Benefit of Cover to your SMOKE unit and to anything hiding behind it, which is the clause most people skim: one cheap squad can shield an artillery piece for a phase. Triggered Alerts (1CP) moves an unengaged infantry unit D3+3 inches when an enemy ends a move within 8 inches of it.
Switch to Smoke Shells when the enemy closes. Once the board has compressed past 12 inches the first two bombardments do nothing, and army-wide Stealth on your own units is worth more. Hold a point for Trench Fighters when the line is finally reached.