>>1795869
NTA, but it's actually a puzzle boss: the Parasitized King Slime doesn't need to be defeated to get a Three Star Clear, and it doesn't ever try to advance to the goal. But if you attack it too much, it will go into it's rage mode, buff all the enemies and then start spamming the gargantuan AOE's it's known for.
The trouble is that even if you know not to attack it, all of your ranged attackers will almost always prioritize shooting it down anyways, and it can get snagged in AOEs on accident. If it's hit one too many times it makes the rest of the level significantly harder to win, especially if it happened early. But the Metal Aliens are only really vulnerable to magic. You need to position your magic casters in the far corners so as to allow them to take on the incoming waves while also being far enough away that they won't attack the King Slime on accident. If you have any Girls who fight in melee that have access to magical attacks (such as Charis or Jinrye) they can also help significantly in this level. Either way it's not as hard as it seems, you just need to be careful is all. Hope this helps anon!