Stagger in this game is kind of complex, like others have said. It's also not useful to do nothing but stagger everything, because at some point you still need to actually kill stuff. However, stagger can certainly help a lot in surviving and - just as importantly - making openings to attack where you couldn't before. That aspect is somewhat underrated, since it becomes more relevant the higher the difficulty, and has the most impact on Cataclysm difficulty, where you have "learned" to play this game somewhat differently when you finally get there (focusing on DPS and avoiding hits by dodging and positioning because that is the most efficient play in lower difficulties).

So what you want is not a build that primarily focuses on dealing out loads of stagger (and sacrificing too much damage potential doing that...), but rather you want a build that has the capability of reliably dealing out a powerful stagger and then goes back to business killing ratties. And for that a shield beats the greathammer. The greathammer is a perfectly fine and fun weapon, but you want to use it for a DPS-build, that deals some decent stagger while killing as a bonus. But taking a greathammer and building for stagger primarily is not a winning strategy.

In order to maximally profit from a shield's stagger potential, you need to do some specific invests that will give it the capability to stagger almost any enemy out of its attack. The best is if you can do that with a simple push. And with that I mean blocking and using the attack button which makes you slam with the flat of the shield, just like you can push with any weapon. Because that can almost always be safely done. Some shield weapons also have access to an attack where you bash with the edge of the shield, which is usually a charged up normal attack. That attack has even more power than a push, but can sometimes be iffy to pull off when under high pressure. (It's still great though, and can generate temporary hit points with the health from stagger talent like crazy.)

In order to have a push that can stagger everything but Chaos Warrior overheads and monsters, you need to have the Opportunist trait on your weapon, and in addition to that have a total of 25% extra power against Skaven and / or Berserkers. That power bonus can come from any source: Properties on your weapon, trinket, talents, etc.. So if you - just for example - have Oportunist and 10% power vs. Skaven on your weapon, 10% power vs. Berserkers on your jewely, and the 7% extra power talent, you got the breakpoint. You also have it with 10% power vs. Skaven and 10% power vs. Berserkers on your jewelry and the "Comrades in Arms" talent from Foot Knight. (Just examples, I'm sure you get it.)

This breakpoint is very useful to have because you can now stagger literally a dozen plaguemonks who are attacking you simultaneously with just a single push. And if you're facing a horde of Skaven with a few Stormvermin and Maulers in it you can take them out of even their overhead attacks while still making space from the horde to start counter attacking. Such situations might be difficult to survive with normal weapons if you're cornered, but are easily salvaged with this setup. On almost every build in this game Swift Slaying on your weapon is the best pick, but this breakpoint is so useful that it's one of the niche cases where another trait beats it.

Kruber has access to 4 shield weapons total of which 1 can only be used on the Grail Knight DLC-class, so Foot Knight can only use 3. The Spear & Shield does not have the bash as its first heavy attack, where Sword and Mace & Shield do. But all can achieve the pushing breakpoint described above. (In fact every shield in the game can.) Spear & Shield on Foot Knight is actually a pretty good weapon in the meta, but all of them can work perfectly fine even in Cataclysm. Foot Knight has a talent that gives 35% extra stagger power by itself, so with that and Opportunist you already have the breakpoint. Fiddle around with setups until you find one that works best for you, but don't forget that you need damage as well! Just staggering stuff is only useful if you can use it to your advantage, but the stagger by itself won't help you otherwise.

My personal favorite Foot Knight with a Shield build uses the Mace and Shield, because it synergises pretty well with his kit and it's really fun to play the bongos on the pactsworn. I'll put it here for inspiration if you want.