Script: https://pastebin.com/cHM1yC00

Forge: forge-1.12.2-14.23.5.2810
Techguns: techguns-1.12.2-2.0.1.2_1
GameStage: GameStages-1.12.2-2.0.112
MobStage: MobStages-1.12.2-2.0.8

Understand now, tnks for explanation

Techguns mobs have their own spawn system with dummy npcs that change their difficulty based on biome and distance to spawn. Other scripts can be incompatible.

I made a crafttweaker addon to solve this problem:

set "Techguns Spawnweight Overworld" to 0 and "Techguns Spawnweight NEther" to 0. this will stop all regular world spawns from techguns. (Spawners in structures still work)

then add all techguns mobs you want with this (https://minecraft.curseforge.com/projects/spawntabletweaker) to spawntables. Then they should work together with scripts like game stages.