I spent a good amount of time in this gauntlet, and though I was extremely weak (6 mask, 1 nail upgrade, no use of tools, no double jump) I still managed to the to the last round pretty consistently. After I went and got the double jump, I got 7 masks, completed the pinmasters' oil quest, and got the tacks and the poison from shellwood. After that, it took me 2 or 3 tries maximum.

Still, I think the last round was really poorly designed. I am not the best at videogames, and I messed up a lot of times, but RNG often backed me into a corner with no way to get out and just wait for the inevitable. The only strat I had left was to keep them both on one side, spam the Tacks and burst down one of them, essentially cheesing the fight.

When the only way to make a fight not pure cancer is to cheese it, then it's bad game design. The only thing you need to fix it is to spawn them with a little time apart so you can at least get a chance to lower one without resorting to cheese.