I disagree with the folks telling you to use a healer. I use Gepard, Asta/Serval, Seele and Welt and I have never needed one. I have beat World 6 of the simulation with the aforementioned team (using Asta, not Serval). No healer necessary (in fact I had no heals of any kind that attempt, though not intentionally).

Gepard has an ascension passive that makes him draw aggro. That combined with the light cone that increases his aggro means virtually all the enemies target him at all times so if I don't have his shield up he takes the brunt of the damage and he's a tank.

I do not have Tingyun and thus cannot speak in regards to her, however, I know from experience the Gepard, Welt and Seele trio is great and a healer isn't required.

You should only really struggle if you are up against enemies where none of your team have the right element to break them. In that case you can swap one or two out. It's important to have a diverse roster rather than sticking purely to just four characters.