@Pocky8 i dont know the mechanic of this game's manhwa, but some game allow you to pick certain talent/build to make healer as a viable tank. Take WoW for an example. Druid class is an excellent healer but they also can be made to become an offtank. But i rarely find an offensive mage built as a tank.
So yeah, healer is the best mage based tank here. He/she only need to focus on his/her HP. And i think thats why the healer in this manhwa is the shotcaller since, like you said, if the healer die the party will collapse.