Hi there!
So, the Apostle itself is not a problem, that is just a random enemy that appears sometimes. The cause would probably be the fact that, after getting your Leader (the character you started with) over level 15, enemies in the first levels become stronger. If that's not the case, then is the regular battle, which is meant to be done around level 10. There is also a powerful weapon hidden in the level that Ame can use to one-shot kill the pirates. Hope this can help you to clear the level!
Viewing post in Dream Labyrinth (Version1.0.5) comments
I should have said that I already found the hidden weapon alongside the hidden gear item yet it seems to do less damage than the pistol at about, hitting for around 320 dmg compared to the pistol's 348 despite the stats saying otherwise. I even whacked a Marine bot with a regular attack from it and it did 0 damage somehow.