Enemy NPCs on GTA V missions are usually configured to be only damaged by player. Probably this was done to give the player more targets to shoot, and avoid allied NPCs to "steal" the kills, but can be not desirable in some cases.

This very simple script avoids this, by making ALL the game entities (this includes peds, cars and other objects) damageable by not only the player, but any other NPC.

Download link: https://www.gta5-mods.com/scripts/fix-npcs-killable-by-player-only
Also used Bodyguard Squads to spawn helicopters (https://www.gta5-mods.com/scripts/bodyguard-squads), and Simple Trainer to spawn bodyguards (https://www.gta5-mods.com/scripts/simple-trainer-for-gtav).