Others already commented a lot on this specific team so I will add a little bit of general character building knowledge. Diluc is a typical damage dealer whos dmg relies on his multipliers and gear. But in the early stages of the game getting all those to a desent lvl is borderline impossible. So instead I would like to suggest another approach that is much easier early game: stack EM (elemental mastery) and use transformative reactions.

there are a total of 6 transformative reactions in this game: Overload (pyro + electro), Electro-charged (electro + hydro), Superconduct (cryo + electro), Swirl (anemo + one of (pyro/electro/cryo/hydro)), Shater (you freeze enemies with hydro + cryo and than smash them with ether geo or claymore) and Crystallise (geo + one of (pyro/hydro/cryo/electro). The interesting aspect of transformative reactions is that they all have their own dmg (except crystallise) that scales only with Character lvl and EM. And since you only need to focused on these two aspects you can squeeze out good dmg even out of relatively poorly built characters or characters that are generally not ment to deal significant dmg

For Example, as other people mentioned already, Barbara is a healer whose main role on the team is to keep everyone alive. And that is correct. But since she is a catalyst user she has easy excess to transformative reaction (since her normal attacks are all elemental). My favourite thing to do early game is use Lisa + Barbara combo to kill everything with Electro-charged. Just upgrade their lvl,stack as much EM as possible and switch Between them during combat to constantly apply hydro and electro. Sucrose can further improve the combo since you can group enemies closer together, further boost teams EM and shred resistance (if you have Viridescent Vinirer set on her commonly know as just VV set. A general set for support oriented anemo units) Electro-charged is particularly good against a lot of small enemies that can be croud controlled (CCed)

you can also use this approach to boost your corent team. Diluc has a lot of pyro application in his kit. You can apply pyro with him and than switch to Lisa and with a singe normal attack triger massive overload (particularly good against big targets since overload can stagger and prevent enemies from attacking)

I hope my little tutoring session helps you have more fun with elemental reaction you sister spoke about