For my mod I am trying to make a weapon that kills anything in one hit. The first thing I tried was giving it a massive damage, but bosses from other mods don't die in one hit. Everything else I have tried doesn't one hit kill anything. Does anyone have any solution?

You could have the damage for your weapon be something along the lines of MAX_INT. Or when your weapon is used to attack something, you could find the entity being attacked and just set its health to 0.

From Item:

    public boolean hitEntity(ItemStack p_77644_1_, EntityLivingBase p_77644_2_, EntityLivingBase p_77644_3_)
    {
        return false;
    }

 

One of the EntityLivingBase is attacker and one is entity hit. Any mod that is correctly written for Forge will be using EntityLivingBase as base for their ANY monsters. Next step is just to get entity hit and set health to 0 (there might be event method setDead() but I don't really remember).