The best thing to do is to try and murder it with a magic attack, so your damage doesn't reflect, but if you have to get rid of the shield, try using your weakest physical attack. In Persona 3, Persona 4, Persona Q: Shadow of the Labyrinth, and Persona Q2: New Cinema Labyrinth, that's probably your default physical attack. In Persona 5 and Persona 5: Royal, pull out Joker's pistol or Morgana's slingshot to eat the damage. There's also the skill Tetra Break, which dispels both the Tetrakarn and Makarakarn barriers. It's not super useful or practical to have because the skill is worthless when an enemy doesn't have a Tetrakarn or Makarakan up. The enemies that can cast a karn-skill aren't common enough to waste one of your limited skill slots on Tetra Break, rather than an almighty attack like Megidolaon, which ignores shields entirely.