Chongyun E gives a cryo infusion. It can be used for breaking the shields of hydro heralds. For hydro heralds, one can also use dendro, cryo bows, cryo catalysts (Charlotte, Wrio), Ayaka.

Besides that Chongyun C2 provides CD reduction. Some teams he can be played in are the og national, freeze (with Furina, Yelan, XQ), fridge, melt.

Some other ideas people have tried with Chongyun are: cryo infused Eula/Shenhe/Dehya/Itto, melt Diluc (mix E with 2 cryo NA), cd reduction for Childe. If you are doing freeze Freminet, maybe Chongyun can pair with him.

On Chongyun and Mika: both are cryo but they seem to be opposites as Chongyun gives cryo infusion whereas Mika is a physical buffer. But they can sort of work together as Mika also gives an atk spd buff. Perhaps they remind one of two figures who contributed to the modernization of a certain country?