This ending requires some preparation and paying attention throughout the game so you don't die.
You will need:
-Zombie class unlocked(zombified citizens are friendly meaning +1 to floor electability)
-No limits mutator so you can remove Fair Game ($1500) and Vocally Challenged ($600) at an Augmentation chamber
-(Optional) Industrial quick start unlocked. I started at slums so i didn't have to pick up every single money drop
-No guns mutator. This will make stubby fingers less annoying and you will die less
-Continue mutator helps as well
-Money Rewards mutator so you have enough money to remove Fair Game and Vocally Challenged
-DON'T pick mutators that spawn zombies on each floor otherwise you run the risk of killing the mayor or election clerk
-Any other mutators of your choice except sandbox so you can get the achievement :D

Now you can start your quest to "legally" takeover the city :D

When you begin each level, try to infect some easy enemies like slum dwellers. You also want a melee weapon asap so you can kill faster. Remember to get as much money as you can and infect as many people as you can. If you complete the big quest it's probably fine to move on from that floor
but if you want you can infect a few more people just incase they all died.

You want to remove Fair Game and Vocally Challenged by the Mayor Village, but preferably before that incase you get unlucky at Uptown 3. I removed Fair Game by Park 2 and Vocally Challenged by Downtown 2 so it's definitely possible. To win the elections the city electability has to be positive so it's okay if you mess up a couple.

Now for the part where you become the Zombie Overlord >:)

If you have reached the Mayor Village with positive city electability and removed Fair Game and Vocally Challenged, then congratulations. You're almost ready to assume the role of Supreme Zombie God-Emperor. But before that, you have to win the election.

1. Give weapons to the Bouncer. You won't need them anymore.
2. Head to a gold building with a Clerk at the front desk. ASK TO RUN FOR MAYOR AND DONT GET THE BADGE FROM HIM. Otherwise, asking to run will bug out and you won't be able to get the achievement.
3. Head to the park
4. Talk to the clerk and you should win the election
5. Talk to the mayor and you should unlock the achievement! :D

Thank you for reading my guide
Feel free to comment feedback or tips :D
Hope this helps you become the Zombie Overlord!