This spoiler free chapter of the guide to The Last of Us 2 has a walkthrough for The Seraphites stage. Ellie is searching for another person responsible for the massacre near Jackson.
Go forward and walk through the revolving door. Use the protruding objects to get to the roof. Move through the abandoned shops and exit through the window.
There are several buildings in the area you can visit (including one with a few clickers). When you are ready to continue your journey, reach the gate, hop on the red container, and jump over to the other side.
Enter through the door and then set the container on the rise. Now get back to the chain and pull it to make the container fly outside. Place it in such a way that would allow you to jump over the net.
Office and mutants
Go up the stairs and break the window. Jump on the scaffold below and then go down to the ground. Use the door to go deeper into the building. At some point, you will see a transformed (hunter) crawling nearby. Follow him and you will get to the big office with mutants. Although it is possible to kill all mutants, fighting them is not recommended - there are too many of them.
Instead, try to sneak around them - stick to the left side of the room and walk forward, taking advantage of, e.g. holes in the walls. If you walk slowly and quietly, you will avoid most confrontations with enemies.
Pass more rooms and try to visit those marked with a plaque (e.g. storage) - they will lead you to the exit. Finally, break the window and run to the emergency stairwell.
Jump on the scaffolding on the other side and go to the open window. Here, in addition to the stalkers, you will also find a clicker - eliminate this enemy first. There are plenty of creeks here - you will be able to move around the rooms undetected.
Your goal is to get to the yellow door. Press the Square button to help Ellie escape the mutants.
Swim a little bit and then pull up to the shelves located above to reach a platform hanging down from a pipe. Go over it, then walk over another pipe and jump to the other side. Grab onto the ladder and go up.
Going through the park - Seraphites
As you walk ahead, you will hear whistling. Walk towards that sound, and after a while, Ellie will get shot in the arm with an arrow. Press R1 to pull out the arrow and prepare for facing a bunch of enemies. You can escape by crouching and going left, or, for example, you can choose a Molotov cocktail and use the element of surprise to kill the group. There is very lush vegetation in the area, so you can crawl/crouch calmly and take out the rest of the enemies quietly.
After dealing with the representatives of the new faction, head towards the hospital. Enter a building marked as Garden Suites. Search the building carefully, then go outside using a ladder.
Move on, and after a while, you will come across another group of Seraphites. You will see the group execute a WLF member. It is not worth intervening -instead, go to the right into the bushes and eliminate the enemies quietly. There are plenty of opponents in the area, you can eliminate them using stealth, firearms, or focus on stealth/bow/stealth kills. The latter variant is best because the number of enemies can overwhelm you.
Keep moving forward while killing more opponents. At some point, you come across a big ravine - jump over it to the other side.
Enter Merci's Saloon and open the red door. A huge enemy with a two-handed weapon will attack you. The enemy is very strong and it is best to kill him with a headshot or, for example, use a shotgun. Soon, his friends will arrive - you have to kill them too.
Run upstairs. A training manual for explosives is on the desk - it unlocks the explosion tree for Ellie. From now on, you can create explosive arrows or upgrade mines. Jump downstairs and walk through the door to the rooftop. Go down the ladder, jump into the water, and swim to land.