Interestingly I found the complete opposite. I couldn't get past the 3rd room (unless I skipped it and just ran straight north to the 4th and died there instead). I found if I killed the 2 in the first room together I always got the sword upgrade, then if I killed the 2 little ones in the next room on top of the big stationary ones I got some kind of upgrade (walk speed, attack speed, damage) but in the 3rd room, no matter where or how I killed things, I didn't get any drops and then I died.
I also found that given the unique mechanic, too many of the enemies were stationary so you couldn't control where you killed them
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Did you ever run east when you get the forking room? There might be some help there :D My idea was that with the stationary enemies you have to think about maneuvering the small ones to them to get the best reward! The sword in the first room is a guaranteed drop in the first room as it gives the most visual feedback instead of random stat increase! Weird it should work if you kill them close enough to each other :think:
My first playthrough I didn't get the sword, I didn't kill them on top of each other and got 2 hearts, which I didn't need at the time. I played 4 times and couldn't get past that 3rd room. I tried again and went to the east side of the room but all I found was locked door. Nothing I killed in that room dropped anything any time I played