And he is not even the boss of the Lab Assault, just a small fry on the way...
Buuut luckily there are a few strategies you can use on him. The problem he challenges you with is his high armor value and the fact that he forces your companions to do inefficient flanking attacks while they still have melee targets.
Stella's Tug of War can help you reposition the shield guy, so his taunt won't affect all your characters. Once you have him on the outer right position Stella can start clearing his allies.
Another possible counter is Akari's Battojutsu. She skips an attack, but deals double damage next turn. Against double damage, even his armor can't fully absorb the blow.
Combine those and suddenly Akari has a high chance of countering with double damage against the shield guy, taking him down in a few rounds.