Also a very fatal blow would be killing the Tyranids main controller, The Hive Mind itself for it controls every single tyranid like a chess piece and targeting it a certain star system. The only problem is, its a suicidal attack, for the reason you have to get past away from the Milky Way galaxy itself and find the Hive Mind (And face an endless horde of Tyranids). If you manage to survive and find it, usually you will die easily since most psykers always goes insane or even die when they find or look upon the Hive Mind, for the fact that its a complete mass of psychic energy and is very complex in structure and its power. Speaking of psychic energy, It can travel through the Immaterium itself to summon a Hive Fleet same as how the Astronomican works. I bet its a massive stalemate if the Hive Mind shows itself on the Immaterium. If the Tyranids lose (They avoid Chaos when they can) and the Hive Mind gets destroyed, the Tyranids start to live on their own which sounds better since you dont have to fight them at a staggering WAAAGH! like scale.