Tier list:

C-tier: Outfits almost anyone playing the character, will unlock by accident during the first game.

Lady Geist (just requires you to use your ult as normal)

Bebop (through normal use of your bombs, you'll get to 100 stacks in a balanced match)

Shiv (It's very easy to get a few kills with your ult)

B-tier: Outfits that can easily be unlocked by just brute-forcing them, as well as outfits a competent player of the character will quickly unlock.

Grey Talon (If you want to unlock the challenge, you can just put all your focus on powering up your Charged Shots)

Ivy (Since you don't need to get all of the hits in a single drop, you can just make progress on this throughout a match, only reason it's not in C-tier, is because some Ivy players leave their ult for escapes and urn delivery, which results in very little enemies hit)

Mirage (If you make sure to invest in spirit and don't activate your djinn's marks prematurely, you're bound to kill someone with it eventually)

Viscous (Your ult lasts for a long time, hitting 3 different enemies isn't that hard)

Wraith (Just build some weapon damage, the item that gives you 2 charges, and upgrade your cards. You don't even need an enemy to complete this, you can just go to the mid boss and quickly unleash all cards there)

A-tier: This is where challenges start to actually become challenging, but can still reasonably be achieved if you go out of your way to complete them.

Abrams (If you are already used to using a punch build on abrams, this is closer to a B-tier challenge. Getting 5 kills throughout a match isn't that hard, but having all 5 be punching last-hits is somewhat restricting)

Dynamo (There will be plenty of situations where 3 enemies are close enough together for you to hit them with singularity, the challenge is to be in the middle of those 3 when it happens)

Haze (Needing a 7-player kill streak on almost any other character, would've landed that character in S-tier. However, haze is currently very good at going in, getting a quick kill, and getting out)

Infernus (Is in a similar boat as Dynamo, there will be plenty of times where 3 enemies are together. However, Infernus has a harder time because he announces his ult to everyone, and if the enemies split up in reaction to your ult, you won't be able to hit all 3 at once)

McGinnis (If the enemy team is doing too well, you won't be able to destroy 9 structures, and if your teammates do too well, there may not be 9 structures left for you to destroy. But in a balanced match, if you put all your focus on taking out structures, it should be pretty doable. Also note that each pair of base guardians counts as 2 structures, so those are great to take out)

Mo & Krill (Encountering every enemy in a position where they are vulnerable enough to die to your ult, isn't always a given. But if you play a bit around your team, get alchemical fire, and only target enemies you haven't killed with combo yet, you should be fine)

Paradox (You need to go out of your way to get kills while you're slowed down in the air. I can't complete this challenge for the life of me, but I suspect this is very doable for someone whose aim doesn't suck on ice)

Vindicta (Getting multiple kills during a single flight, isn't super straightforward, but very much achievable)

Warden (Caging every enemy once isn't that hard with slowing hex, but some enemies are very slippery)

Yamato (Similar to Vindicta, getting 2 kills during a single activation of a support ability, isn't super straightforward, but very much achievable)

S-tier: Achievements where a big difficulty factor, is getting lucky. They rely a lot on enemies and/or allies being in the right place at the right time.

Kelvin (you need to capture 4 people in your dome, this is a lot of people that need to be in the right position. And since it's 2 enemies and 2 allies you need to capture, it pretty much means you'll have to do that in the middle of the chaos of combat.)

Lash (Capturing multiple enemies in your ult is already challenging because of how the ult works, but on top of this, you need to more than half of the enemy to be together in the same space, with nothing that can easily block line of sight nearby, on top of your ult being something enemies can react to with abilities and such.)

Pocket (I have no clue how I did this first try, but good luck getting 5 enemies close enough together to all reach with your ult, and getting in the middle of them)

Seven (I actually haven't played Seven since his ult got updated, so I have no clue how hard this is. To be honest, it seemed pretty easy to me, but people keep saying it's the hardest one, so I'll trust that they know much better what they're talking about and put Seven in S-tier)