Dark Shaman vs Monsterslayer Shield
In situations like this, games generally favour effects that add restrictions over effects that enable. ("Can't trumps can", as Magic: The Gathering players paraphrase the second Golden Rule of that game.)

It's not very clear from the wording which effect adds a restriction until we rephrase them in terms of damage: "Damage from Dark Shaman can't be blocked by shields" vs "Monsterslayer Shield can block any amount of damage". This makes Dark Shaman's ability the winner.

In short, it doesn't matter how much damage Monsterslayer Shield can block because it doesn't block any.

Keep in mind I have never played the game, but your question has been asked on Board Game Geek before, and all three posted opinions concur .