Diluc

Diluc is a top tier Pyro physical damage dealer with a claymore that can also attack pretty fast. His elemental skill can be used 3 times within an activation, each hit dealing Pyro damage and can be used together with his normal attacks. Even though it’s at a 10 second cooldown, if you space it right by using 1-4 normal attacks in between, by the time you are done, the skill will be up again. The cherry on top is the fact that his energy cost is among the lowest of the initial release of characters, making it quite spammable. Against a group of enemies, you’ll be able to use his elemental burst left and right.

Diluc has a lot of Pyro attacks in his kit, so he works best when you focus on his Pyro elemental damage, instead of building him as a physical damage dealer. The best artifact to use is the Crimson Crimson Witch of Flames artifact set, which is almost made just for Diluc. Use 4 piece Crimson Witch set to get the full effects, and get Pyro DMG bonus on the cup, and crit rate/damage on the crown.

Out of the two available craftable claymores, the better one to use for him is Prototype Aminus, since the secondary increases attack. Serpent Spine is also a very solid choice to use, if you claim the claymore from the Battle Pass. Because it gives crit rate from the secondary, you can use crit damage for your artifact crown piece, increasing your potential damage by a lot. The best in slot weapon for Diluc is the Wolfs Gravestone. It's a powerful claymore in general, and always a good weapon for any character that can use it.

Diluc is the only character that can do elemental combos really well with other characters. This is because his elemental skill can apply Pyro 3 separate times, possibly triggering any elemental reaction 3 times as well.

Chongyun is an amazing character to bring with Diluc, since his elemental skill lets you apply Cryo in a more controlled way. Diona is a good replacement if you don't have Chongyun.

Then there's Xingqiu, with his water abilities to proc Vaporize. Using Xingqiu and Diluc's elemental burst, you can keep triggering Vaporize as long as you use Diluc's elemental skill in between 2 normal attacks.

There's also Bennett, a character that's very good on any team in general, but bringing him with Diluc will give you the Pyro elemental resonance for an easy attack boost.

Beidou can be a nice addition to your party, especially if you are not very good at dodging enemy attacks. Her elemental skill can put out high numbers if done correctly. You can even use her elemental skill on burning grass, that you create with Diluc, to deal high damage in an instant. Her elemental burst also works well with Diluc's burst, triggering an Overload reaction every few normal swings.

Normal Attack
Performs up to 4 consecutive strikes.

Charged Attack
Drains Stamina over time to perform continuous slashes.
At the end of the sequence, perform a more powerful slash.

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

Performs a forward slash that deals Pyro DMG.

This skill can be used 3 times consecutively. Enters CD if not cast again within a short period.

Releases intense flames to knock nearby enemies back, dealing Pyro DMG. The flames then converge into the weapon, summoning a Phoenix that flies forward and deals massive Pyro DMG to all enemies in its path. The Phoenix explodes upon reaching its destination, causing a large amount of AoE Pyro DMG.
The searing flames that run down his blade cause Diluc's Normal and Charged Attacks to deal Pyro DMG for a time.