Chongyun

Chongyun is an exceptional Cryo elemental enabler. He definitely adds a lot of value to the team since his first skill essentially converts all melee attacks to Cryo, for easy Cryo application. This means that you are not locked to using him as main DPS. On top of that, his elemental burst has a fairly low energy cost and recharge rate, while also dealing a good amount of damage.

Prototype Aminus is a great F2P option. The ATK% it gives is generally helpful since you will be relying on both Normal Attacks and Skills. The weapon's passive is also quite powerful especially after gathering a group of enemies, but has a long cooldown. If you don't have the materials for Prototype Aminus, Debate Club is a similar 3-star weapon you can get from the Gacha.

For players with Wolf's Gravestone, it can give Chongyun a massive ATK boost with its base stat alone. The passive is just icing on top, giving even more base ATK boost.

DPS Chongyun mostly relies on triggering Shatter. Because of this you need a mix of Crit and ATK main stats with some EM substats. Ideally you should get ATK% on your timepiece, Cryo Damage on your goblet, and Crit Rate/DMG boosts on your crown. The final set you should be aiming for is a 4-piece Gladiator because as of now that's the only 5-star set that really fits DPS Chongyun. Interim sets could be anything that strengthens his Normal ATK regardless of element; this means Brave Heart and Martial Artist. Particularly, building a 4-piece Berserker allows you to make full use of a Crit DMG crown and focus your substats on Crit DMG over Crit Rate.

The best weapon for a support Chongyun is most likely Sacrificial Greatsword. One thing about Chongyun is that his Skill has a longer cooldown than his Burst, which is kinda bad for building up your Burst especially if he is the only Cryo character on the team. Meaning it would take a long time for you to actually use his Burst as a Burst support. Sacrificial Greatsword fixes this by immediately ending your Skill cooldown so you can get energy orbs again. Additionally, the Energy Recharge stat makes charging your Burst a lot faster.

Even if you don't have The Bell, any greatsword you would put on a DPS Chongyun should suffice for a support Chongyun, especially Prototype Aminus for F2P. The main thing is you still want to build up his ATK since you'll still be using his Skill and Burst to deal damage, and Chongyun won't be the one triggering the Reactions.

If you're only using Chongyun to apply Cryo then switching out, and using his elemental burst when it's up, you're better off using 4-piece Noblesse Oblige set. Energy recharge set (2 piece The Exile + 2 piece Scholar) is also pretty good with Chongyun's low Burst cooldown, if you want to have a elemental burst spam fest.

The main and substats you want on these artifacts are mostly ATK. Your cup should still be Cryo Damage, while your head piece can be Crit Rate or ATK%.

DPS Chongyun needs a teammate with constant Hydro application to consistently Freeze and Shatter the opponent. Xingqui is the best for this purpose, with his Elemental Burst applying Hydro to enemies even at mid-range. Another great option is Barbara, additionally giving Chongyun constant heal. But you will need to get much closer to the enemy to apply Hydro with Barbara's Skill.

Support Chongyun is very effective at applying Cryo to the enemy so one of the best team comps for it would be in a Melt team. For this you need a Melee user such as Bennett with a Short Skill cooldown. With Chongyun's Skill active, apply Cryo to the enemy with Normal Attacks then immediately follow up with Bennett's skill to be augmented by Melt. Xiangling is another candidate for this with her Skill and Burst being able to continuously apply Pyro, but not faster than the Cryo application of her Normal Attack.

After applying Cryo with Chongyun's Skill or Burst, switch to an Electro user and trigger Super Conductor. The one that immediately comes to mind is a Physical DPS Fischl. The reason why we don't use melee characters such as Keqing and Razor is because their Normal Attacks are converted to Cryo DMG due to Chongyun's Skill, and Super Conductor applies a Physical RES debuff.

Normal Attack
Performs up to 4 consecutive strikes.

Charged Attack
Drains Stamina over time to perform continuous swirling attacks against all nearby enemies.
At end of the sequence, perform a more powerful slash.

Plunging Attack
Plunges from mid-air to strike the ground below, damaging enemies along the path and dealing AoE DMG upon impact.

Chongyun strikes the ground with his greatsword, causing a Cryo explosion in a circular AoE in front of him that deals Cryo DMG.
After a short delay, the cold air created by the Cryo explosion will coalesce into a Chonghua Frost Field, within which all DMG done through Normal and Charged Attacks by Sword, Greatsword and Polearm-wielding characters will be converted to Cryo DMG.

Performing the secret hand seals, Chongyun summons 3 giant spirit blades in mid-air that fall to the earth one by one after a short delay, exploding as they hit the ground.
When the spirit blades explode, they will deal AoE Cryo DMG and launch enemies.