Shadowland teams that work for me every time:
Mordo, Kaecilius, Wong for early Blast Rumbles - I use Kaecilius' leadership for physical safety. I've tried using him and Mordo solo, but I've only been able to win with the both of them together. Wong is just there for the additional team bonus. I switch to Mordo and use a skill rotation of 2-5-2-4-2-3 - basically, I just keep returning to 2 between 5 and 4 as much as possible because it has the fastest cooldown. The match barely lasts a minute.
Lincoln, Daisy, Lash for any Blast Rumbles and Relays - Lincoln's leadership boosts his own damage significantly and all his skills keep everyone stunlocked. I open with his Protective Dome skill (3) and just do a random skill rotation from there until everyone is dead. Zero thought goes into it, that's how easy it is. I could probably save Daisy and Lash for another floor.
She-Hulk, Totally Awesome Hulk, Hulk for early Combat Relay - I only have She-Hulk and TAHulk fully built, so I just tag between them frequently with little thought put into their skill rotations. She-Hulk's damage leadership and their combined team bonuses are the winning factor here. I'm not using this team much anymore because her leadership is more valuable on later floors. This is an easy win.
Luke Cage, Iron Fist, Daredevil for Combat Relays - I use Luke's leadership for safety and switch frequently between him and Fist. I try to make frequent use of Lukes 4th skill because it stuns enemies. Since I T2ed them both recently I've been saving them for later levels in the teens, but at T1 they're super effective on early floors. If I decide to buy Danny's new uniform I'm probably gonna break up this team.
Deathlok, Ms. Marvel, Lady Sif for early Combat Relays - This team is a little harder to win with, but still doable. Deathlok is only there for his damage leadership. I tag out frequently between Kamala and Sif, relying on Kamala's 2 and 5 skills for stunning enemies. I haven't landed on a skill rotation for either one yet.
Wiccan, Vision, Hulkling for Blast Relays - Wiccan is the leader and damage dealer here, with the other two just there for team bonuses. I use a skill rotation of 1-3-5-1-2-4. Wiccan is really fragile and easy to hit if I'm not perfect with with skills, so sometimes I'll die and have to finish the last enemy with Vision, who's only partially built. Still an easy team to win with overall though.
Ronan, Thor, Ghost Rider for early Universal Relays - Ronan's mostly here for his leadership, though I do switch frequently between all three characters. Ronan's 5 skill is good for herding enemies into corners when pressing at a diagonal.
Singularity, Jane Thor, Robbie Ghost Rider for early Universal Relays - Mostly the same deal as the above team, but a lot easier, with Singularity there for leadership. I use a skill rotation of 4-3-5-2-1 with her before tagging out. Jane and Robbie are both really great on their own, so when I eventually T2 them, this team is dead.
Shadowland solo characters from floors 1-20:
FLoki - I hate Loki, but there isn't anything he can't do. He's just a chore to play with.
Crystal - I love using her on later floors because her crowd control ability is insane. I team her with Gorgon and Karnak, but it's not even necessary. Skill rotation: 5-3-1-2-4.
Mantis - I soloed Iron Fist Boss on floor 13 this week (the one with the absurd dodge ability) without even trying - and it was my first time using her.
Hawkeye (Kate Bishop) T2 - A guaranteed win on any floor that allows the use of a speed character. I like using her against the team of Lash, Nico, and Wiccan, rotating between skills 2-4-5.
Gwenpool T2 - Same as Kate Bishop. I like to team her with Squirrel Girl (in case she dies) and Modok (for the team bonus), but it's usually not necessary. Skill rotation: 5-3-2.
Captain Marvel T2 - She's literally unkillable. Just make sure to maximize her damage output because she's a little slow to win. Skill rotation: 4-5-3-2-1.
Black Panther T2 - I use him on early Combat Rumbles against Agent 13, Black Widow, Elsa, and Gamora. I just have to make sure I take out Agent 13 first because she's the most deadly. Skill rotation: 5-2-3-1-4.
Black Bolt T2 - I save him for later stages where applicable, like the fire relay (Angela/Ghost Rider/Red Hulk/Goblin). Skill rotation doesn't matter so long as I open with his 5th skill.
Wasp T2 - I save her for the Hulk Rumble. Bubble, buff, attack, attack, attack, repeat. Easy win so long as you don't slip outside the bubble. Team bonuses are helpful but not necessary.
Ancient One T2 - Also useful for the Hulk Rumble. Even at T1 it's an easy win so long as you keep his invincibility and healing going. Also good for the fire relay.
Elsa T2 - I save her for Loki Boss. Needs a Guard Break Immunity obelisk to protect her 5th skill from getting interrupted. Skill rotation: 5-1-3-4. Easy win.
Silk T2 - I use her for Rocket Boss. Rocket's healing skill makes this fight longer than it should be. I could team her with the other spider people, but it's not necessary. Sometimes I never get hit. Skill rotation: 1-3-5-4-2.
Moon Knight T2 - I save him for the Male Relay against Iron Patriot, Falcon Cap, and Captain America. This team does a stupid amount of damage, but luckily Moon Knight is self healing at T2. Skill rotation: 2-4-3-5.
Songbird T2 - I save her for the Female Relay. I use her 1 and 2 skills to keep them stunned and the 4th skill for burst damage between normal attack volleys. Slow and steady for the win.
White Tiger T2 - I save her for the Hawkeye Boss. Keeping her shield up and using her 1 and 5 skills to keep Hawkeye stunned is the key to winning.
Solo characters for floors 21-25:
Hulkbuster - The only use I can find for him in Shadowland is against Venom on floor 21. Apparently his uniform is key to keeping Venom stunned while being hit. I rotate between 1-2-3 until it's safe to use 5, then 4. Been teaming him with Iron Man and War Machine for the bonuses until he's at T2, but they don't do anything otherwise. If I still have She-Hulk available, I'll use her leadership here.
Proxima Midnight - I save her for the Villain Relay on floor 22. Her damage output is so absurd that all I have to do is make sure I make good use of her iFrames and keep moving so I don't get hit. I use Hela's damage leadership to speed things up, but it's not even necessary.
Doctor Strange - I save him for floor 23 against She-Hulk and Jessica Jones. I was originally using FLoki on this floor. Strange destroys, obviously.
Yellowjacket T2 - I use him on Floor 24 against Luke Cage. I don't have an attack ISO-8 set on him so this fight takes longer than it should, but it's still an easy win.
Sharon Rogers T2 - The Daredevil fight on floor 25 is annoying because he does that stupid roll move that cancels your active skill. I just wanna get it over with at this point and Sharon completely shreds in seconds.