That army is part of a wildland. The Great Scrapyard i think it's called.

They definitely do have defense against blunt, though it may be less than their other defenses. Regardless, there are a few ways to deal with heavily armored units. In order of ease:

1. Spears. They partially ignore armour. Stack up a huge bunch of spearmen with as much armor as you can get, and go a-stabbing.

2. Elemental staves. Fire ice, (and lightning for pariden). They're in the magic tree. research them, and make troops with them (ideally on horseback so you can keep backpedalling and shooting). Elemental attacks entirely ignore armour, and they will make iron filings out of any big metal creature. Staves cost lots of crystals to make though, best get mining.

3. magic weapons. As above, elemental attacks work well. there's a huge amount of enchanted weapons in the game that have some amount of fire/ice/electric damage. Find them and put them on your champions. Also wear the rings and amulets that give elemental attack bonuses.  If you're far enough in the magic research tree, you can even get these things to put on your regular troops. The Lightning Pike is probably the ultimate anti-armor melee weapon, for regular men.

4. Magic. fireballs, flamedarts, shadowbolts, blizzards, etc. magic ignores armor.\

5. Shadow weapons. Specifically the shadow dagger and broadsword. they're a pair of uncommon weapons which ignore significant amounts of armour. Also the Shadow Strike traits on the path of the assassin, can eventually get you a hero with 100% armor penetration.