Note that the amount of enemies and diffilculty scale with the amount of players in your game. So in therory, Solo is easier then Co-op. Except Solo is like playing Co-op with Iron on. Co-op you get nice little respawns if you die. Solo? BACK TO THE CHECKPOINT WITH YOU!
Now, I leave you with two images. These two images will give you tips.
Tip #1. Use Stealth. Assassinate as much as possible. If you can quietly dispatch enemies, they will not be alert to you.
Tip #2. Is it not painfully obvious?
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