When Zombies have taken refuge in a concrete bunker, or earthen bunker with timber support, Zombie defense tactics may present an offensive challenge.
However, revisiting some tank warfare tactics of the past may provide instruction. If you can get your hands on a tank and suitable ordinance, then the challenge may be made easy. Otherwise, some improvisation may be in order. One possibility is to shoot the ordinance into the Zombie bunker opening from a distance such as with an RPG-type weapon or other shoulder-fired grenade launcher. The other is doing it the good, old-fashioned way. Just watch some old World War II movies for instruction. When attacking German concrete bunkers, a suitable, but dangerously heroic, method was to run upon them from behind, and on top, and toss a satchel charge into the bunker opening. This works, too.
But, ideally, like I said, you need to get your hands on a tank and suitable ordinance. In a perfect world, you will have rounds of high-explosive and rounds of canister. Here is what to do. Drive the tank up to the Zombie bunker opening and ram the barrel of the cannon through the aperture of the bunker so the muzzle is just inside the bunker. Fire a round of canister into the bunker. Your canister round should be 700 steel ball bearings packed in high-explosive. This will shred most of the Zombies in the bunker. Your next round is a round of high-explosive. This will finish off any Zombies that are still showing signs of, um, life, and should collapse the bunker on top of them, effectively entombing them. This was the method used in Vietnam against Viet Cong bunkers, when tanks could get to them, as described by Ralph Zumbro in his highly instructive book, Tank Sergeant. These Zombie defense tactics are actually quite offensive in nature.
So remember, kids, canister then high-explosive for bunkered-in Zombies.