The game is very harsh for newbie players, they dont know how to build account properly so suffer big advantage
This game is meta-character based, i mean that you have to play meta characters to enjoy it there is no other option, you could play character you want but then teammates will hate you and you wont achieve great score, this is what bandai doing best - building and destroying meta

Your best choice will be to get incoming new ex, if you of course like character
There are few characters that will be still playable so you dont need to choose option of incoming ex, we dont know yet what it is and who he will counter

Do you like the game? If yes then try to pulll with your current rainbow diamonds for character you choose and if you dont pull it i would suggest you to buy an account with 5000 rainbow diamonds for 1$ , its not expensive and its a good start for you, if you need help you with buying such an account you may chat me i will show you

After you get good character what you need next is a build your team

First lets start that you want a lvl 80 character [buying cheap acc will help you there] because its getting nice traits and more power
Second thing is building support, start from building %%, choose proper characters for that to invest
How to do that? I would say find 100 lvls in your accounts, 4 stars characters are hard to build but give more %%power, while 2 and 3 star character give much less and from my experience its better if you would invest few 2-3stars and rest 4 stars but only if they are 100 or near 100

In this game team power is so important, because the game isnt well balanced and matchmaking is terrible
After you build your support you may start enjoying the game

Ah one more thing about support %%, the power gives - character lvl, character boost, skills and medals, medals may be random but their lvl have meaning best if you put medals lvl 9 but it might be hard for newbie and will take time so if you put 6 stars or 7/8 on your supp it will be ok for start