The arousal loop is solvable by just waiting, the poison wears off after 5 minutes, so too many defeats won't lock you out. The tricky bit is killing off all the walkers in the groups with the flies so they don't block the flies from chasing you, and then surviving the chase across the whole map with 2-4 flies chasing you.
Edit: Okay, difficulty setting affects spawns. Hard mode has flies patrolling the corridors, so I think that'd be the best place to get the scene.
Edit2: Yeah, boosting the difficulty was the right call. Enemies now need 4 hits to kill, but in addition to the 2 flies in the loop around the teleporter, and the two at the noise ambush north of that, there are 2 more new flies that patrol between these two areas. Do a couple of quick runs to pick up a heal, carbine and max ammo, exiting and saving then reloading that save - this lets you keep the items, so you can go into the proper attempt with 3 heals. You could pull the flies north or south, but what I did was run north, use the carbine to gun through the walkers that spot you and keep running east, making sure i had the 4 flies (i'd picked up the 2 patrolling ones by this point) following me. Past the ambush is a loop, so I ran that to turn around without dashing through the flies, then lead them back the way I'd come and then south to the carbine teleporter with the last 2 flies. The heals ensure you can survive a few more mistakes that cause the flies to hit you. Since gunshots aggro enemies, you can use them to pull foes at the teleporter and ambush from greater range.