I'll echo that you should make a team for each element to start with. It might be worth mentioning what 4* (and even 3* units) you have. Some of them are extremely good and might help you round out your teams well.
I'll just go element by element for my 4*+ units, then. We'd be here all day if I was listing my 3*s too.
Fire: Mikoto, Sarisse, Maribelle, Euden, Yue, Xuan Zang, Sinoa
Water: Orsem, Luther, Karina, Elisanne
Wind: Ku Hai, Addis, Musashi, Ranzal, Pia, Aeleen, Eleonora
Light: Albert, Annelie, Ryozen, Yachiyo, Luca, Amane
Dark: Orion, Sazanka, Norwin, Cleo, Edward
Healers seem to be a weak spot for me in basically every element.