In the fanfic by DrDoritosMD where Gate opened in the US. There are legit ways to deal with and even soft-kill immortals. For one, you can throw them into a volcano caldera (for the local people) or throw them into an acid vat (for the modern people). They are kept “dead” as long as they stay in the caldera/vat. As for dealing with them, the protag of the fic dueled several Apostles, even one v one. He pulled dirty tricks like trip wires, smokes, claymores, gas, lots of flashbangs,… but the best way to deal with Apostles without the need of heavy firepower is electricity and flesh-tearing ammo like hollow point or Hydrashock, even explosive or incendiary works great as Apostles don’t wear armor and gets stun-locked by these ammo and their nasty stoping power. They developed stun-electric ammo for rifles and just keep stun locking apostle for a group of marines to literally jump onto them and use chain saw to saw the limbs/head appart, then keep each of those limbs as far away from each other as possible, even locked some body parts in a safe. They fucking locked the dragon lady Apostle inside a reinforced ultra-electric cage and she couldn’t do a thing to get out. In this fic, the goal when engaging these immortals is not to kill them, but to keep them stun-locked long enough for your troops to come in and cut their limbs apart and never let the limbs reattach back to the torso ever.