Subnautica is intended to be and continue to be a sandbox game with no weapons. This is why there is only a knife in game, and no guns or other weapons. Adding a grenade is be a cool idea, but I doubt that the devs would add it in.
Why multiple things per acheivement? Just have one objective per acheivement, like an acheivement just for cooking a peeper, not a peeper or a garryfish.
