After years of playing Destiny, I finally understand how to make things harder. All you have to do is increase the number of enemies, increase their damage output, and decrease player effectiveness.
That's literally all there is to making games difficult. I feel enlightened. I feel I can make a really good game now with this knowledge. Thank you, Bungie, for teaching me this. I will try to follow in your footsteps and make things as grindy and bullet-spongy as possible. ^^