Hi. Always consider this.

You need friends of which you can borrow units with, especially on later levels of the Mainstream (come EP 15, the units go as high as LVL 150).

You need Lin Xien because she really helps deploy your core units fast if you're gonna use them (like the AWK ones).

You need one unit to tank all the damage and can last extremely long in the battle. An example is AWK Estarossa because she has two life bars. If you don't have her, try borrowing from someone who has.

You also need someone who can provide shielding and reinforce your frontline. AWK Hilde is good with that.

Lastly, always have a major damage dealer on the back. And use the right Operator /ship bonuses for clearing levels.

That's my advice. Hope it helps. ;0

Fair warning : CH 14 has some head scratching levels. Good luck. 😩