Taking the Wall
This is a fairly straightforward introduction to the combat. Start out by charging into the brawl. For now, just pick off fighters on the edges with a few quick blows and help Foltest's men seize the first section of the wall. The area ahead will reach a stalemate, so Geralt will need to drop down and get creative with a ballista.
Walk toward the tower and look to the left at the bottom of the staircase. There should be some scaffolding that you can drop down on. Drop down with a mouse-click when the context is right, and then repeat it when you reach the end of the platform.
Combat is going to get a little tricky. Even on normal, it can be hard to handle groups alone. Look at your signs and start by setting a Quen shield around yourself. You do this by hitting "Ctrl" to bring up your list of signs, and then use "Q" to cast. You may want to take a Swallow potion too. You start out with four, and you should be able to get somewhere that you can meditate.This will do wonders to keep them from nickle and diming you down. Also remember to check your gear. You may have something better in your inventory that isn't equipped yet. Even the default loadout has a better steel sword waiting in the inventory.
Start in the back corner. The basic knights are no problem. Just try to hit them with a quick attack, and then follow it quickly with a strong one to finish them off. Don't let them get behind you, and roll away from blows. Parry won't work if you're surrounded (not yet anyway), so try to evade as much as possible.
Move toward the ballista, but do so by sticking to the right. There are two crossbowmen waiting behind the walls, so flank one and take him out quickly with a few slashes. Run or roll to the other and try to repeat it.
There's a well armored knight here, so fall back and keep some room available to retreat. Again, Quen helps a lot. You may have some luck with Aard too. Stick to hit-and-run tactics to pick off the weaker knights. The shield carriers and armored knights are easy by themselves.
For the well armored ones, just wait for them to start their strike, and then ready a parry. Parrying their blow will deflect most of the damage, and it will stagger them. Hit them with a quick attack immediately, then try to follow with a strong one for some solid damage. Rinse and repeat until they're dead. Note that this is a great time to quicksave, and wait for your health to regenerate.
The ballista here is a bit easier than the one above. Start by approaching it, and choose to ready it. Click the mouse repeatedly to beat the QTE and pull the string back. A few basic knights will spawn, so run out and attack them as you did before. Beat another QTE to aim the ballista, and then fight another wave of guards. One should be armored, so watch out. It should be the same as before though. Just pick off the support and parry the knight.
Run back and fire the crossbow to bring down the gate.