This is a custom run where the only enabled game mode is the notoriously difficult Holdout mode where you have infected pouring in from everywhere and you need to eliminate them all while defending the (not so smart) NPC ally.
I certainly wouldn't recommend going for Holdout on Grounded as things can go south very fast and getting an S rank is near-impossible, but even on Grounded completing the mode is doable which is what I wanted to prove with this custom challenge.
I played as Tommy as he is my favorite character and because his inability to dodge adds an extra layer of challenge. I also wanted to include all 5 Holdout maps in the run. As for the boss, I selected Rattler Captain which is widely regarded as the most difficult one and I personally think so too.