Find an alternate way into the compound, then find Germain and kill the thugs.
Assassin's Creed: Unity guide - Sequence 5 Memory 1: The Silversmith - Kill the Thugs
This mission's optional challenges involve using your latest toys and gadgets, so they're something of a pleasure. We still have a few missions to complete before we have access to Arno's entire bag of tricks.
- Lockpick four doors
- Drive two Brutes berserk
There's a pretty easy route to your goal. Stay up high and head for the northeast corner, where you should see two open windows. There are a couple of Extremists nearby, but they're really not hard to get past.
Once inside, immediately take cover against the west wall and use Eagle Vision to mark the three Extremist guards. One of these can be dispatches with a cover kill if you wait patiently, after which it's a doddle to sneak up behind the other two one by one and dispatch them. Leave the one by the window for last.
Now that you're safe from detection, you can start on the optional challenge; just wander back to the first room and pick the lock. You don't need to pass through this door, though - whether you returned to do so or not, go west and stop in a hiding spot just before you reach a pair of Brutes. You can sneak up behind them and put them down silently as you did the guards in the last room.
This paragraph only applies to those trying to earn the optional challenge. Look for another door to pick the lock on, then head through to spot an Artifact on the wall. On the other side of the Artifact room there's a third pickable lock, but there's a guard nearby so be careful. Once you've picked both doors, go back to the area with the first pickable lock.
To reach Germain, head through the white double doors on the western wall. You'll be rewarded with a cutscene.
Kill the Thugs
Protecting NPCs is always a pain but this can be achieved quite easily if you work methodically to clear guards. To kick things off, go into cover at the door with three guards outside. Mark them with Eagle Vision so you can follow their patrol route, then take down the two on the right side with silent kills. You can then drop the lone third guard with ease.
Continue onwards but take cover when you reach the top of the stairs. There's a Brute down below, so if you're after the second optional challenge, send him Berserk. He'll take out a lot of the guards for you, so wait until he's down before you work on the rest of them.
The next bit is a bit tricky, requiring speed and decisiveness (and being much easier if you've already unlocked the double assassination skill), so read ahead before moving on. When you move forward you'll spot an Extremist in the next room, but he'll move away as soon as you spot him. You have to sneak along at hyper speed to take him down with a silent kill. Immediately move forward and take down the Brute and the other guard. Any hesitation will result in the three guards splitting up and becoming much harder to take down.
The last lock you need to pick for the optional challenge is in a side room near the desk, so head in there before you move onto the next set of guards. This bunch of four guards is a perfect opportunity to put your Berserk blades to work again, so target the Brute and let him do his work. Once everyone is down, don't head straight out the front door - there's a door to one side you should take instead. Outside, you'll be in a perfect position to flank the two guards on the balconies and air assassinate them.
With everyone dead, head back to Germain and clear off the baddies around him to secure your exit.