I can't stop watching... oh, wait, here's a review
Alright, I got one complaint. That Homer Simpson scream was pretty funny, but then it sounds like you used the same one a second time a couple minutes later. It seemed out of place the second time and wasn't as funny.
That's it. That's the only complaint in the entire world that I have for you.
Your choice of music was better than ever this time around. Back to Mad, Star Fox, and the Sonic & Knuckles final stage remix... I had to go track down that last one because it was so awesome (and completely appropriate given what game Mecha's from.) Not only that, it sounds like you changed the intro for that song back to the original Sega version - and I agree with that decision too. Your sound effects have always been great, well-timed, funny, never out of place and this was the same here. Sound: 10/10
The fight scenes were incredible. Mecha Sonic's rampage on the Axems was jaw-dropping and the individual skirmishes going on were creative, distinct, perfectly choreographed. To give each Axem so much personal flair to their fighting in only two flashes is impressive, and the heroes themselves - well, I just love how you use them. Violence: 10/10
I'm a Smash Bros fanatic, so of course I recognize some of their special moves they've used, mainly Luigi and Yoshi. Yoshi being one of my favorites, I love you to death for letting him stick with the main cast and even get in some cool moves. Good call on the sprite change for him, which I think must be from Yoshi's Island 2? It distinguishes him from his tribemates and makes him look a little more modern too.
Your humor always makes me laugh, and makes me realize how much I love the Mario Bros. You've even made me fond of Shadow, who as extra Sonic character #42 always seemed a bit forced to me. He really plays the "Vegeta" part here and I wouldn't mind seeing you push that to the forefront at some point, maybe get frustrated with not being the strongest, although if he carries on too much it might wreck the pace of this series.
I laughed when Shadow does things like kick Mario, I'm not sure if it's to save Mario or to get him out of the way but it's funny. The Over 9000 thing got me laughing too, especially since the scouter displays Pac-Man of all things. And Luigi's cowardice is classic. Him getting a blue shell was perfect, allowed Mario to use him without Luigi having to "do" anything, and brought in that whole Koopa-shell-skateboard thing I loved in Mario 64.
Which reminds me, the teamwork in battle is a treat to watch. It really helps explore the fighters' behavior in a way a generic beat 'em up sprite movie wouldn't. It may be the single best thing about your fights IMO, other than their sheer overall quality.
I'm dying for part 7 but take all the time you need. I respect you and your work way too much to whine at you or harass you about it, and I know quality takes time. I can't even imagine where you'll take things next, or what surprises you'll have waiting for us.
Since most of the people from the big brawl are gone, things will probably start drifting back towards solo fights or smaller team matches... I'm sure you could continue the huge 15-person battles with enough brainstorming and sprite-hunting, but I really think you can step up your one-on-ones to the quality of the brawls as your skills continue to grow.
Focus on the characters, man -- if Wario or Waluigi ever get into a fight, knowing how ridiculous and crafty they are, I bet a fight with them in it would be a real treat to watch with all the surprises they could throw in. That's my advice, not that you need it at all. XD
One request: Bowser's been beaten, and his sprite seemed like it may have been clumsy for you to use compared to guys like Mario, but I wouldn't mind seeing the big guy again, fighting or not. Maybe just a small role, maybe Mecha walks all over him or some other terrible stroke of luck hits him, whatever it is it'd be funny.
There's not a thing I'd ask you to change about this series. It's my favorite flash ever, and I thank you.