Fox should win this matchup, but not by much. I've spent some time playing against Mii Brawler after losing some close matches to one as Megaman and Sheik -- Mii Brawler is a strong character, but has significant weaknesses that can be exploited.
Stay in the center of the stage at "golden" percents and he can't kill you with helicopter kick -- it's literally impossible unless you are near the ledge / he carries you off stage. Once you're out of the golden %, you should be able to DI towards him for d-throw/d-tilt and only get hit with u-air/nair. At higher % you can DI away and he cannot combo from d-tilt or d-throw. At this point, the game is "don't get hit with f-smash, side-b, or up+b", which isn't super difficult. Scary, but not terribly hard.
Use 2331 - impact blaster, wolf flash, twisting fox, regular shine. If your scene doesn't allow customs, get a better TO.
Around 40%-60% or so depending on DI, d-throw -> wolf flash is a true combo. If it is timed/spaced correctly, it spikes Mii Brawler. This kills the Mii Brawler when he is off stage. I repeat: you have a guaranteed kill combo at certain % out of d-throw. You get to give Mii Brawler a taste of his own medicine. You'll have to read their DI to know whether to jump before you side-b or not to get the spike, and outside of the true combo range they can air dodge (but be virtually unable to punish you because, you know, mii brawler offstage), but this is a super useful tool in your arsenal. If you condition them to air dodge, you can hard call it and jump out and fair spike them. <3
If you find yourself with your back to the ledge, you are in danger. You can cross him up with a dash attack or a full hop bair on his shield. Tomahawking behind him and grabbing is good. His punishes aren't going to net him a KO unless you're at very high percent and every Mii Brawler's reaction is "GRAB AND COMBO". This will take you out of golden % if you're in there.
Around 0-10% or so, d-throw combos into fair -> u-air depending on their DI. This puts them into bair combo % range.
Around 30% or so, Bair combos directly into dash attack -> u-tilt. Follow up from there.
Around 40% or so, u-tilt combos into short hop fair spike -> jab lock -> u-tilt -> repeat. They can DI out of its second iteration if they're wise to it, but even if they do you can still hit them with nair.
So if you have a perfect game, you can win in two engagements: a grab at 0% followed by fair-> u-air followed by bair -> dash attack -> u-tilt -> sh fair spike -> jab lock -> repeat until kill % -> twisting fox / u-smash depending on stage.
Mii Brawler can be hard to grab if he doesn't want to be grabbed, but if you get him and get an f-throw off at mid% it can give you a jab lock with your impact blaster. The instinct is to do a running u-smash, which is the easiest. If you are technically capable (I fail a lot, so I u-smash >_>), you can actually do a dair pop -> fair spike combo and end it with a d-smash that sends Mii Brawler out at a vicious angle.
Edgeguarding Mii Brawler is
cake. If he air dodges below the stage, he should probably die. Your bair out spaces his up+b and as long as you aren't timing it / spacing it to where you'll get stage spiked, he should die after you bair him. His up+b often lasts so long that you can punish it on reaction, but given its super short vertical distance you can just say "now" and bair him in the face. If you're super scared you can do a full hop nair off stage and hit him that way, but that typically doesn't kill.
If Mii Brawler isn't at kill %, don't bother edgeguarding offstage. Just bair him from the stage if he goes to high with his up+b / d-smash as he's coming up.
When Mii Brawler is on the ledge, standard getup is his best option. FUN FACT: Mii Brawler has a longer getup animation than other characters. Same amount of vulnerability, just takes a little bit longer -- it's like Bowser. So alter your timing a bit. I like to fade away nair to tap him off stage. Running off stage and auto cancel bairing onto the stage is super tight frame-wise, but if you do it you can immediately land and u-tilt, dash attack, or dash grab. If your reaction time is fast enough you can theoretically guarantee a hit, it just might not amount to much.
2xJab works decently against Mii Brawler. I'm bad at it, but jabbing him to the ledge -> d-smash is a great way to set up for an edgeguard.
If you dash past Mii Brawler and pivot f-tilt his shield, he can't punish with anything unless you space it horribly.
If you u-air and miss, Mii Brawler can either not punish you effectively at all or can kill you with his up+b. Learn the difference.
If you space something incorrectly, Mii Brawler can kill you with his purpley over-b dash thing or an f-smash. At higher % Mii Brawler can punish many attacks on his shield with d-smash OoS -- it has a sweetspot in front and sourspot in back, so crossing up helps, but he can turn around OoS and d-smash and still sweetspot. At higher %, your job is to just run away.
Impact blaster can almost be spammed in this MU. Take on a little extra %!
If you f-throw Mii Brawler off stage, immediately impact blaster. If he spams jump, he loses his second jump. This should kill the Mii Brawler.
There aren't many opportunities for easy vertical KOs with your twisty fox up+b in the MU due to his air mobility. He'll miss his up+b sometimes, but it's hard to catch him with your up+b in the air.
Footstool combos into your twisty up+b. :3
On walled stages, you can sometimes shine spike Mii Brawler by just spamming shine. No joke, if you shine them next to the wall, you just fall with them and shine again. After 2 shines they might be able to get out (I have gotten 3, but 2 is safe), so ending it with a fair or dair spike is likely better. Both of you can wall jump, so you need to take him lower than he can recover.
D-tilt at around 100% or so combos into u-air but doesn't kill. It makes me sad, but it looks cool. Sometimes I try d-tilt -> twisty fox up+b but it isn't guaranteed and rarely works.
If you fall in love with d-throw -> wolf flash, know that spiking them on stage means you will get punished. They bounce and can immediately act. Try to hit them with the sour spot to send them upwards when on stage.
Your standard kill is likely going to be dair -> u-smash at higher % if you're not edgeguarding. You should be aiming for at least one bair edgeguard kill per game on average in a 3 stock match. If your TO runs 2 stock, find a better TO.
There's really nothing to avoid from Mii Brawler other than being grabbed in the wrong location at the golden %, being raw f-smashed/up+b'dd/over-b'd. Don't spam over-B off stage in a predictable manner or they will up+b where you WILL be and kill you. If Mii Brawler tries to edgeguard you off stage, just go really low. He can't edgeguard off stage unless he gets a hard read or you let him.
