How are you meant to kill the first shielded unit?

I played some more, and with some more understanding of the game mechanics, this is not as daunting as it appears at first.

Firstly, damage dealt is the simple calculation of the attacker's attack stat minus the defender's defense stat. What makes this shielded unit so formidable is that his shield grants +3 defense, giving him a total of 9. That means, to deal more than 1 damage at a time, you need to have an attack stat of at least 11. Grind until your attack stat is high enough to be useful, or attack with magic (which isn't affected by defense), in order to prevail.

Another thing that helped out immensely was actually running away from the battle. By killing all the other enemies on the battlefield, then retreating, that allowed me to swarm this single enemy on my next visit to that battlefield, since he was the only enemy that remained.

Use the time between your retreat and your next attempt to grind at the first shrine. There are 2 enemies in there, and they respawn every time you visit the shrine proper or exit the dungeon. After 3 trips back and forth (12 quick battles), the shielded enemy in the main mission posed no difficulty at all, even without a mage.

So this seems to be a common predicament for a lot of players at the beginning of the game.

From that thread there are a couple of suggestions on winning this battle, and why it's in the game. (Bold bits are my thought on it)

• You don't have to, but are kind of expected to grind a little bit. I can say though that you should probably go ahead and promote your villagers. Leather shield guys is kind of absurd; I think that was designed to be a teachable moment about items but without a mage the guy's kind of a pain. (This would suggest grinding to level up to beat him, or bringing along a mage if you don't have one)
• I had to box him in from all directions and rely on RNG that he missed because everyone died in 1-2 hits from him and dealt 1-2 in return. (Boxing him in and hoping for RNG help may be a tactic, although this came from the question poster who said they struggled to win this).
• You can pretty much beat the game without grinding if you know what you're doing. The leather shield merc can be a problem for new players, as suddenly' there's this one super strong enemy. Ideally you would have promoted your villagers before fighting him. (This again suggests promoting before the battle)
• The leather shield mercenary isn't a problem if you have an archer and a mage, honestly. (Again someone suggesting you use a ranged unit to help win this fight).

Generally though this fight seems to have bother quite a few people, mostly because of RNG supposedly playing a role, but grinding to promote or using mages/archers in the battle also seem to be valid options.