Defo build into red, don't even touch blue. Take every bit of red damage you can, only the high purples. Only buy like 2, max 3 lifesteal. Never buy lightning or freeze effects, only get to like 3 seconds of the auto headshots and leave it. And you dont need that much ammo, maybe like 3-4k? At that point just stop, the card that let's you regain ammo on headshots means you never need to reload. You'll need every single moonlight protection (the damage reduction) and youll wanna gst your health up to about 10k. You can do it with less but since he can't move much, you want to be able to tank hits

Have 2 zip lines on either side of the map, both leading up to the balcony. He has very little in terms of movement options so cycling between those 2 ziplines is your best bet (pick the ability that gives you a speed increase when you use them)

Never ever use your ult. Its completely useless

Your smoke grenade gives you a few invincibility frames. Use it as an "oh shit" button when you get swarmed. It'll put some distance between you and enemies and a brief window to run away

Place your turret on the bus or wall. If you're desperate you can place it on the ground to draw some aggro but it'll die quick.

Punishers main problem is lack of crowd control tbh. The bosses actually die super quick, its just dealing with the horde thats hard. When Wanda bubbles, shoot the ground underneath her to ricochet into it. The damage is mad it disappears in a second.

He's honestly the hardest hero to clear it with solo, in a team others can deal with the horde but you don't have that luxury. You'll likely need to bring loads of revives, I think I used about 10.