There are many, many ways to get easy PP in Blackest Friday.
This guide/video merely describes/shows an accidental exploit I discovered when trying to play Blackest Friday from level 1.
The meat part of this video starts at 2:41:33, timestamped in the description. (sorry I can't get the timestamp to work in this steam guide embed)

The video was not created as a guide. And this guide kind-of sucks as a guide.

Basically, skipping the other steps in this guide,
-get to the mall,
-complete the go-kart trial, and die after receiving your reward of 5000pp. You will reload the last checkpoint, which was immediately after completing the go-kart trial, and receive 5000pp each time. This can probably be repeated infinitely until reaching the next checkpoint (yellow marker) but im not sure.
There may be light spoilers.

To be honest, I am not going to lie to you; for me, starting Blackest Friday from Level 1 wasn't too bad.
It only took about two hours to get to the mall.
Each time you die your total kills and PP gets saved, so you're not really starting all over again each time you die.
I was level 3 when I reached the mall and reached level 4 as soon as I had control of my character. I upgraded Melee for critical hits and maybe health or something I forgot.
If you just keep your eye on the prize and tell yourself youre going to die alot, and eventually level up, to buy more perks, it's really easy.

^The Miami Boardwalk Raceway is located in the Miami Boardwalk section of the mall, west of everything.
I do not know if you can run directly there as soon as you start Case 1, you may have to clear some objectives. There is a checkpoint involving a Hotel. I suggest you finish that first. You can sort of farm the Hotel checkpoint and get 100PP per human soldier, and then just die before finishing it to reload there to farm more human soldiers.
Anyway after you clear the hotel you should be able to get to the Miami Raceway.

Complete the Go-Kart trial, and die after you are rewarded 5000PP.
For me, the game created a checkpoint, and each time I died after that I was reloaded into that checkpoint, and kept getting rewarded 5000PP each time.
I didn't want this, and did not realize it was happening until i gained about 5 levels.
After realizing what was going on, I decided I would NOT re-do Case 0 again just because the game is broken, and tried to reach the next checkpoint. I was even able to get the reward after completely closing the game and re-launching to continue the story.
That is all for this guide.

There are MANY OTHER PLENTY instances similar to this where a checkpoint is created and a reward is given upon loading.
Aside from those, PP is pretty easy to farm if you decide that getting it "naturally" is not ever going to happen in Blackest Friday, and youre going to need to farm ANYWAY for achievements if you are going for achivements...
To easily farm PP some of the other "legitimate" methods are using combo vehicles, normal vehicles are good for this too like the Civilian or Military Humvee, or the gang burrito, or the ambulance, tractor, etc, big heavy vehicles...
Aside from using vehicles for farming, there are probably other methods, i really don't think it is that hard once you get out of the mall... and even in the mall there are vehicles that can be used for light farming to an extent.
There are very many other actually useful guides for PP farming; this is just a small tip in-case you are stuck or in-case you want to gain a few freebie levels before reaching the next boss at an early part of the game. Again, I didn't want this, but in the long-run, so many levels later on are basically free just from driving around and entering stores and picking up collectibles...
Blackest Friday really was not that difficult for me to "beat" and get the ending.
Most of my playthrough was kicking zombies when forced to fight zombies, and using guns and explosives (and vehicles+vehicle weapons) on humans/bosses. I REALLY don't think Blackest Friday humans can be beat with melee weapons alone, which was disappointing to me.
I hope this helps anyone who might be struggling a little in that first part of the mall, but I think you can do fine without this guide.

If for whatever reason you are stuck inside the mall, and you have gone past this particular checkpoint, I might suggest using some of the vehicles in the mall to farm PP (just run over zombies), entering every store that you can, rescuing the survivors that are standing on top of kiosks if those are available (i forgot)....
The mall Maniac boss for me was kind of bugged so I was able to cheese that boss pretty easily. If the mall Maniac boss doesnt get bugged for you, and you are still struggling, i can only suggest to farm PP and buy perks, or try to run to the spot where you can climb onto the upper levels of that part of the map.
I think it is dumb that the mall Maniac boss is tied to the story and you can't really skip it.

I must add that after beating the game from Level 1 on Blackest Friday, there are SOOOOO many moments and instances in the game where the game cheats YOU.
Because of this, I don't think anyone will feel that blatantly using this method as an exploit (even to get to level 100) will really chastise you.