Well I'll do a compress guide of all I know, to help the beginners for what units you should use to make a good mono team, which is the meta right now, also I'll give a small explaining about the unit can do besides the role like Silas she have a great Atk break of 30% .

The team normally should be compose of 1 Tank, 1 Healer, 1 Damage Dealer and 1 support, but you can change the support to something else like another damage dealer, a sub-tank or even another healer this spot is flexible. From the best to the worst on the spot here(Like Garcia is the best Tank, but you can survive with Alven) is the Fire Team:

Fire Team - Tank - Garcia, Wu Lian, Alven, Chloe(limited).

Healer - Nefeeru

Damage - Mao, Lorelei, Lotus Moon, Rabis(free)

Support - Petralia, Alven, Silas

Garcia is the Best Tank he can heal himself, reduce damage he receive and boost the HP of the other fire units.

Wu Lian is a Tank/Damage, she can do a lot of damage with her counters and her passive is very good self boost.

Chloe is Taunt Tank, she can avoid attacks, have some mitigation and can do some damage.

Alven is a Tank/Support he support is base on boosting the team, one thing that is special about him is that he can boost any element so is 25% HP and atk boost and he's Soul Spirit give more 15% HP, Crit rate and Crit damage. He is very easy to get 10 copy.

Nefeeru is a Healer/Support she buff the attack of team with skill 1 and with passive(Fire unit) also can increase Presence points but you usually you don't have points to put there.

Lorelei do a lot of Damage, she only looses to Mao because of her SC, besides damage she can Dispel the enemy buffs.

Mao is great Damage and she can reduce the CD from all skill of the party which is great but have a low chance.

Rabis is Damage, that is all she can do.

Lotus Moon she can break defense up to 30% which is great.

Petralia she is a Support/Healer but the Healer is just single target, she can reduce the CD from all skill(self) and passive increase Defense by 30% and reduces their attack intervals also to any element, means that you can use her in other team also like alven.

Silas is a Support because she can break up to 30% atk which i think she is the only unit in the fire team capable to do that and most of the Breaks in game is 15% to 20%.

That is the fire Team if you guys have any suggestion leave the comment below Ill add here and if you guys liked I'll do the same with the other elements following Water, Wind, Dark and Light(too much information about light here in the reddit already).

Have a good day always.

Edit: did some changes with base on what Smilesrck said, read his post under for extra informations.