Bullcrap, it was Aqua Man and Freeze Man.
You're joking about Freeze Man, right? It's true that there's a whole bunch of stuff going on during the battle with him, but he does sissy damage to you unless you touch him, which you've pretty much got to go out of your way to do. The most challenging part of the Freeze Man pea shot fight is having to put up with that nasty noise they try to pass of as "boss music" in Mega Man 7.

Aqua Man isn't even the toughest Robot Master to beat with the plasma in his respective game. That happens to be Astro man. Aqua Man you can pretty much dodge by anticipating where his wave will rise or fall. Astro Man is 100% luck. Unless you use search missiles like you're supposed to, he's invulnerable to pea shots during his entire Astro Crush phase. Dodging Astro Crush is also about luck. If he decides to keep using Astro Crush instead of dropping to the center of the screen and sending out his two satellites, well then you're flat out screwed.