Outfits are unlocked by completing different tasks such as beating the game, finishing the Arena: Colosseum mode and many others. You can see how to unlock them all by going into Settings > Gameplay and do what it takes to get them all. MOST of them are not hard to get and there's just one or two that might require some effort from the player but the challenges are very doable and should not give you trouble unlocking them.
Now notice I capital said MOST because there's one outfit that will test all your skill and patience in this game: the McFly outfit! The requirement to unlock it is to beat speedrun mode in under 1 hour. If you look it up on YT you'll find several speedrun videos of this game but most of them get close to 1 hour by the time they complete it. I've watched a few of those speedruns and they make a solid decent run with advanced time saving killing techniques and still finish at the 54, 55, 57 min mark so this mode leaves a very small room for error. I'm not considering speedruns containing glitches and skips, only normal runs from beginning to end having to fight all the enemies and bosses.
The thing with this mode is that 1 hour is roughly the time it takes for an average player to make a "perfect" run so the devs were very precise on that time and some players might even consider unfair I'd say. The reason why the time to finish is so tight, considering you make a solid speedrun, is because there's a lot of waiting whenever you reach a new area and have to eliminate all enemies before proceeding. I mean, you'll fight a whole bunch of enemies before continuing and even if you kill them immediately as they spawn you still have to wait for the next wave to spawn and that consumes quite some time, which is key here and when you put together all those seconds you lose just waiting for enemies to spawn and ships to drop more enemies these seconds turn to MINUTES, a few minutes that will determine if you'll succeed or fail.
But don't let this keep you from trying and eventually succeed. It IS very challenging to master a 1 hour speedrun but practice makes perfect. I personally ALWAYS struggle with speedruns no matter the game or the game genre for that matter, speedruns always prove to be a challenge for me and make me spend a lot of time trying to master it and getting the par time. And that's funny because I see a lot of comments from people that watched the same video guide as me and say they got it after a few attempts. In my case I'm not exagerating when I say I tried HUNDREDS of times on some games until I pull it off (not the case in this particular one). Keep at it and you'll eventually get it.
Anyway, I diverted a little bit from the topic at hand but back to it: before attempting the speedrun mode know that if you're playing on Xbox there's the crashing issue. The game WILL crash while you're playing this mode if you're doing it on console. From what I found it differs from player to player on which part the game tends to crash. For me the game kept crashing on the "Mothers Womb" chapter right after the part you kill the 4 ANGELS each one with a hosts' pile under them. I've talked to the most recent winners of this achievement and some of them got it playing on pc and they said the game didn't crash at all, not even once, so apparently the pc version can sustain this mode to the end without crashing. It is possible to complete it on console, you'll want to empty your system cache every time before playing this mode to make the crashes less often but still it's not a 100% crash free and occasional ones can happen. It seems on Xbox X the crashes are less likely to occur.
I'm leaving this video guide showing a 57:38 min successful speedrun that I used as reference. You can find other videos with a shorter completion time and some of them are finished with several minutes to spare but these are skillful players with advanced killing approaches and some risky strats that can get you killed. They come close to death on many occasions.
When it comes to speedruns I always look for the tightest successful par time because these are the ones where normally the player makes some mistakes and has a simpler less advanced gamestyle and techniques but is still able to pull it off. I like that in a speedrun because I feel I have room to fail at certain points which will cost me some time but not enough to ruin my run completely to the point where I have to start over. Before my successful run I was having an even better one where I was 1 min ahead of the guy in the video but sadly the game crashed.
Then I retried and managed to beat it!
My time was 58:40:140.
Just a few tips that will save you some precious seconds:
1- If you defeat Nerve then restart the speedrun you'll start with his blade as your melee weapon instead of the pipe. The blade deals more damage killing or incapacitating many creeps of the inicial levels with just one swing.
2- If you come to a new area where there's a checkpoint and you're facing tough enemies being low on health like half a cross I strongly recommend you just die immediately and restart from the checkpoint with full health rather than trying to survive and end up dying without making to the next one. If that happens you'll have to kill all the enemies again to the point you died and as I mentioned above waiting for the enemies to spawn or drop from the ships consumes precious time.
3- You'll notice this player clear most areas with the VIS CARVER melee weapon using the spinning attack. It's a very strong and helpful attack that wipe out most enemies instantly and you'll be using this ability a lot. Just keep an eye on the durability of your weapon. You don't want to waste it all up and wait a few seconds for your supply drop to send a new one before you can continue spinning killing your foes. To prevent that from happening the best thing you can do is whenever you reach a new area while you wait for the enemies to drop from the ships or spawn call the supply drop, just be careful not to be squashed by it. Grab the fresh blade before the timer runs out and if your previous weapon is still solid you can switch freely between the 2 without having to worry about calling a new one while you fight.
Other than that there's not much more I can say to make your life easier. Just follow the video guide, try to memorize the levels layout and where the enemies spawn to minimize your overall time.
All credit goes to Michael Lux for recording the video.
I hope you manage to beat the par time challenge itself and wish you good luck with the crashing issue. GO PUPPY!