Invest in more tools early on

Mirage places a much smaller emphasis on skill trees than the last few Assassin's Creed games. When it comes to expertly sneaking past guards, Basim's tools are the real star of the show, so I recommend focusing on unlocking more as your first priority. You're given one tool unlock for free, and I recommend grabbing the sleeping darts first. The rest are unlocked through the middle skill tree. It takes around six skill points to complete the set—throwing knives, smoke bombs, traps, blowdart, noisemakers—and it's a worthwhile investment before worrying about eagle or focus mode upgrades.

Visit general shops to buy outfit dyes

There are so many free ammo chests on the rooftops of Baghdad that I rarely had to visit general shops (represented by a bag icon on the map) to restock on tools, so it took me way too long to notice the other main thing these merchants sell: outfit dyes! You'll earn dyes for your default assassin garb through the main story, but other outfits have three alternate dyes that you'll find in shops and in treasure chests.

Sell your trinkets

It also took me too long to notice there's a second tab in stores to sell stuff. All those trinkets you've been looting from chests and pockets can and should be sold all at once for a big payout. 

Don't sleep on sleep darts

This being a throwback Assassin's Creed, I expected to rely heavily on throwing knives to maintain stealth, but my favorite tool ended up being the basic sleep darts. They don't have the range or permanence of a throwing knife to the head, but they do have subtlety on their side. Putting a guard to sleep will "turn them off" for around 10 seconds. That's just long enough to sneak past their purview, and if another guard discovers their sleepy friend, they'll just wake them up instead of going on high alert (which makes stealth a lot tougher from there on). If you are out for blood, sleep darts are also a great backup if you're out of throwing knives. Their range can be upgraded after unlocking Tier 1, and a later upgrade even makes them lethal.

Smoke bombs are a combat cheat code

My second most-used tool after sleep darts were easily smoke bombs. These can be used to temporarily blind guards from a distance so you can pass unnoticed, but they're actually best used in combat. Throwing one at your feet (which you can do with the quick use command) in the middle of a fight will instantly incapacitate every guard around you, and susceptible to a hidden blade execution. This tactic is so overpowered that it almost feels broken—combat basically comes to a halt as Basim either makes an escape or kills everyone in the smoke cloud for free.