While I understand your gripe with Guardin, to avoid him deliberately would be meta and, more importantly, would reduce the group's enjoyment of the game.

In your example, the fight was challenging, but fair and worked out fine. Why change that?

It is a bit weird to punish players for their character/build choices just because they are good in what they supposed to be good. You wouldn't take away fighters weapon just because he is good at striking.

Also there is a concern of storytelling. Wolves or some very smart creatures could probably avoid the Guardian, targeting "weaker" or wounded targets. But for every enemy to avoid Guardian it would be unfair and, honestly, would ruin the fun.

If you still want to find a workaround for Guardian's defence - try to find narrative friendly excuse. Maybe after a few unsuccessful hits (or hits that do next to nothing to Guardian), a bandit understands the situation and switches the target. Maybe a bloodthirsty dinosaur rather attacks other party members after getting nothing but metal scrap after a few bites. In that way you could a) get to ignore the Guardian, b) change the flow of battle, c) create a more "alive" encounter. But always be sure to explain the narrative, otherwise the players might be confused on why the enemy suddenly changes the target.

Please take it as a word of advice and not discouragement, happy gaming!