The easiest way i found to get this achievement wasn't mentioned anywhere, so i made this guide.
It uses a door exploit, like another way i saw, but way easier and safer. These kind exploits have been patched sadly, but you can just change your game version to before the fix (pre 18.2).


First of all, close the game.
Then go to your Steam library, right click on House Party.











Then Properties -> Betas -> anything before 18.2, for example 17.3







Close the dialog, and wait for Steam to finish the download.




Start the game.

Load a new save, go upstairs and open both the master bedroom door, as well as the bedroom bathroom door.

Now for some useful things, but nothing necessary.

You can build up combat stamina before the fight by running around, and masturbating, but it is not necessary.
You can also do more damage by drinking the 6 Natty Littles found around the House, which are in these locations:

-One behind a couch in the living room
-One behind a speaker in the party room
-One on a shelf in the garage
-One on a high shelf on the left as you walk into Master Bedroom
-One behind the hamper in the Upstairs Guest Bathroom
-One behind the desk in the Study

Also i recommend taking one of the painkillers (behind the master bedroom bathroom door) for faster combat.

Now for the fight itself.
https://www.youtube.com/watch?v=YGhX9-Clug8
Enter combat mode (space).
Hit Frank (left click).
Go out of combat mode (space).
Run for your life upstairs, into the bathroom.
Position yourself behind the door (close the door) in front of the shelf.
Now, open the door again. (you should stay kind of inside the door)
Go into combat mode again (space).
Happy hitting Frank, while he can not hit you. (I've been hit once in over 30 min fighing from there)
https://www.youtube.com/watch?v=X8v95rA46bY
You must, however, not hit him K.O. while inside the door. When his health reaches a point of maybe 1 to 3 hits, go out of combat mode, close the door, run towards the bathtub.
Frank will open the door, and run towards you.
You need to run past him, and go outside.
Hit him the last few times via normal means. (running around the pool, or garden)




EZ