Team building
Building a proper team becomes really important around Chapter 7. Enemies then not only will start to have more Combat Power than you, but most importantly, their teams won't be made out of random characters and instead they will create annoying and synergetic compositions.
That's why around Chapter 6 you should take a good look at the characters you've pulled so far and decide on the direction you want to go. Do keep in mind that the characters you are Ascending past Epic aren't always the same characters you will use in your team - the game might give you a lot of copies of a character you don't like or don't want to use, but yo still will Ascend her.
The image below shows all faction bonuses possible:

Your goal at this point should be to build a team that can at least make use of the first Faction bonus - and do that you need to have 3 characters from the same faction in your team. This will provide you with a 10% boost to ATK and HP to all your characters.
And here's a list of characters that everyone will receive for free during the first 2 days (we skipped Seeha and Mephi as you will need around a week to get both):
Good synergies
Below you will find a list of synergies we found that make some characters perform much better.
Honglan and Talia/Vivienne
Honglan's damage output and speed manipulation make her the strongest character in the game currently that makes the game much easier. Still, she has a glaring weakness when you want to use her as your main carry - she's slow, so it takes a long time for her to cast her skills. This is why you pair her with Talia or Viv (or both) as they give her more mana and and speed buffs, and allow you to work around her weakness.
Mica and Seeha
The most famous synergy. When you play Mica or Seeha alone, they simply suck, but when you pair them together, they become a murderous duo. Mica can hold the frontline while dealing damage around her and her Ultimate deals damage to whole enemy team and stuns them, which gives you a few precious seconds to recover. As for Seeha, both her instant healing and healing over time become so strong, she can easily keep the frontline alive by herself. Just keep in mind that if you don't use her ultimate, she will only heal allies close to her.
Also, since both Mica and Seeha are Beasts, you just need to add an another Beast to reach the first Faction bonus - but more about it later.
Jacqueline and Velanna
Jacq teleports to the enemy backline at the start of the battle to wreck havoc there, putting bleed on enemies. Then Velanna's ultimate will deal more damage to the backline and stun them as she becomes much stronger when attacking targets that are already Bleeding.
Naiah and Mephi
Naiah's damage output is a bit lower than other pure damage dealers because the Sleep debuff attached to her skills is already game-changing. Still, when you pair Mephi with her who lowers the whole enemy team DEF and increases their chance to be critically hit it turns Naiah into a monster. Mephi herself also brings respectable DPS and a 6 second stun.
Good early-game teams
Here are some good early game teams that make use of the synergy above (but not always). We didn't use Adrianne or Ayame in any of the teams as they are harder to get and will be harder to raise.
Undead + Human
Velanna is the main carry of the team - her damage is honestly absurd and you don't need to run a secondary DPS with her often.
Jacq's role here is to teleport to the backline and murder the Casters/Supporters hiding there
Petra is the best early-game tank and she works really well when fighting enemies above power as her selfhealing and damage is based on their HP not her (via her Main skill). She's super hard to kill!
Claire is a sub-tank that is more of a disruptor - she will jump to the backline and take some heat off Jacq and Velanna.
Catherine will shield and heal allies and often save them from dying thanks to invincibility.
Fairy + Humans
Viv's damage output is crazy and she's the main carry in the team, but you can also use Mephi as the main damage dealer if you have more dupes of her and then Viv will fill a more supportive role. Chloe is decent tank early (and she becomes even stronger later) that can taunt the whole enemy team and ruin the day for enemy Assassins - she can also keep herself alive pretty well thanks to her passive.
Beasts + Human
Both Dora and Mica can act as the main carries here, so pick who you like more.
Great late-game teams

Team 1 - Wabbit-God Team
This is the current most popular meta team that makes progressing through Chapters 16-17 a breeze.
The whole team revolves around abusing Honglan speed and mana manipulation to mess with enemy formation and their skill synergy - often you even prevent them from using any skills. What's best about this team is that any of Honglan, Talia and Vivienne can be used as the main carry here. So even if your Honglan is lower ascension, she can still fill more support-like role.
As for the formation, you often run 1-4 with Chloe as the only tank, but keep in mind that your Chloe needs to be beefy enough - Origin and level 200 is the requirement for Chapter 17. In the earlier Chapters, you can get away with weaker Chloe though.
Also, while Chloe, Viv, Honglan and Talia are the core of the team, the last spot is a flex one. I'm using Catherine there as her healing, shielding and invincibility helps Chloe survive longer, but you can use anyone else here really - Naiah, Claire, or Jacq are the ones I often substitute in this spot depending on the enemy formation. Adrianne also works well in the flex spot - especially when you need to pull someone from the back.
If you want to see this team in action, check the video below - it showcases Honglan in the more supportive role:
And here are two previous meta teams that still work pretty well.
Team 2
Team 3
You can use Jiho instead of Jacq if you need more healing.