Haha, why you wanna choose her ? (Cause she's 9 tail?)

Have you spent your initial 250 rubies? Cause the early main target of this game is to finish your Masteries first.
I messed up at the starting point but I just continue it and now I'm quite happy with my line up 

Your friend list is already full.

Go for masteries !!

That's your first task 

Your friend list is full.

Add me IGN: Antt00

Yes.
Baijiao doesn't stack with Eileen/Victoria while Dellons's doesn't stack with Lina/Karma

Nope. Back liner are usually dellon, Shane or jupy.

Do not syn 5* element is not cost effective, the most u syn is 4* element. Those old timer mostly do it with light or earth element because too many. New player should just keep all the element as there is so many hero to build.

Alternative way to get element, buy the 3* Element Selector from CR Shop, or re-enter Daily Dungeon on the particular element day. 3* (normal) cost 150 ruby per-retry and 4* (hard) is 300 ruby.
Lee Jung is a keeper, he is worth lvl40, the only hero can reduce team skill cooldown at the moment.

Yes, nia is still a relatively good hero to use with her recent revamp, if you don't have a time reducer hero yet, she is a good hero to use in that regard.

In the past nia was much needed to clear world 5 hard mode because of the shield immunities there. So nia would just do a time jump to remove to shield for you to clear it faster.

But now with her revamp she has piercing so that would make her better to clear that floor faster as well. Same would go for Ruri if you have her. Cause their skills are rather similar effect just that Ruri would have more damage than nia.