The one-shot gun in Kaiju Paradise appears to be a VIP-server-only item, and the wiki notes that similar restricted weapons are obtained in VIP servers through the :give command rather than in public servers. Public videos also suggest that some weapons can drop from presents, but that is shown as a chance-based method and not a guaranteed “one-shot gun” unlock path.

What to do

  1. Join or create a VIP server.
  2. Try the server command system, especially :give, since the wiki says VIP-exclusive items are obtained that way.
  1. Check whether the gun you mean is a specific VIP weapon, because Kaiju Paradise has multiple guns and some are limited to certain VIP variants or events.
  1. If you are talking about a random drop weapon, look for presents in-game, since videos indicate some weapons can come from those drops with low odds.

Important note

I’d be careful with “one-shot gun” wording, because the game seems to have multiple firearm-like items and at least one Golden Weapon that is public- server functional but VIP-variant exclusive. If you want the exact item name, I can help narrow it down from the weapon list and describe the safest legitimate way to get it.