South Park: The Fractured But Whole
36 Achievements
25-30h
Last Kid Standing
Win while 3 Allies are downed at the end of combat
33.8%
How to unlock the Last Kid Standing achievement in South Park: The Fractured But Whole - Definitive Guide
If you would like a video guide I will leave mine below.
Defeating the enemies (besides Professor Timothy) turns them into allies, meaning that you have more than 3 allies (about 7 total). I just fought the fight and lost at least 3 allies, but won with 3-4 surviving allies and it gave me the achievement.
This is very easy to do if you're actually trying for it. It may come naturally/accidentally during the story as well (most notably during the second Doctor Timothy fight since you have way more than three allies). If you don't get it during the story, fight the hobos at the playground and kill all but one of them. Let the other one knock out three of your characters, then use the fourth to finish him off. Worth noting that The New Kid has to be alive at the end, so he can't be one of the three downed allies or it won't unlock.
For this trophy, you must have 3 allies with you when you enter combat (so there are 4 of you total). The easiest way to do this is to start a fight against regular enemies somewhere in the town and get it down to one enemy left, then let him kill off 3 of your allies before finishing him off with your final character. Once you have won a battle with 3 allies knocked out at the same time at the end of it, the trophy will pop.
Good luck. I hope this helps!