The Seven Deadly Sins: Four Knights of the Apocalypse episode 21 followed up on the first meeting of the titular Four Knights in episode 20. While they all may have disagreed and ultimately failed to capture Pellegarde, the fight against the two reborn Commandments went much better than anticipated, owing to everyone trying to be on the same page.
While Gawain may have hogged most of the fighting and got on Percival's nerves, her ego was somewhat kept in check this time than in her prior appearances. Though still shaky on teamwork and fully getting along, especially when it comes to Gawain and Percival's dynamic, it seems like the Four Knights are getting where they need to for ending King Arthur's reign of terror.
Four Knights of the Apocalypse episode 21: Knights vs. Commandments
King Arthur's motive for reviving the commandments

The start of Four Knights of the Apocalypse episode 21 is a brief retelling of the original Sins vs. Commandments conflict, and King Arthur's reasons for bringing them back and giving them power.
His reasons are purely selfish - they are powerful demons, and they'll sacrifice themselves to atone for their villainy in the original series. He likewise brainwashed the two, frying their minds if they go against orders or remember their true selves, which leads to Melascula's downfall.
As long as Liones is destroyed, the Sins killed or the Four Knights defeated, Arthur doesn't care what happens to the two. This is obvious given his lack of interest in the Four Knights, even when the Dark Talismans were defeated by Lancelot or when someone as powerful as Ironside was stopped at Sistana.
In fact, Arthur even laughed off Gawain being a Knight of the Apocalypse but still decided to assess their abilities himself, watching the battle closely.
For someone who seemed interested only in securing a wife for himself, to the point where he tortured Ironside over it, the fact that he's only starting to pay attention in Four Knights of the Apocalypse episode 21 is pretty impressive. It's also a warning, given how much the Four Knights struggled against Pellegarde, that things are only going to escalate from there.
Galand and Melascula, diversion?

Four Knights of the Apocalypse episode 21 continues to show the audience the reborn Ten Commandments, Galand and Melascula. The two of them sliced massive carvings into Liones, which cut the city in half and killed several people and Holy Knights. They are major threats that the regular knights of Liones are powerless against.
Given that it took the original Seven Deadly Sins some time to defeat, they naturally began their rampage looking for them. Meliodas himself even considers for a brief minute going down to fight them, before one of his advisors points out how suspicious it is that they avoided the castle and bothered to make such a huge spectacle of destroying downtown.
As Meliodas and his advisor deduce, the castle and Sins are not the real target. The attack is a diversion to attain Bartra's visions, either to kidnap the elder or steal his powers and the second cloaked figure that accompanied Melascula is on the way there. Meliodas races to deal with the second threat, claiming the Four Knights will deal with the reborn Commandments.
Knights vs. Commandments
All Four Knights gathered to face down the two Commandments. The Commandments compared them to the older Sins: Escanor, Meliodas, Ban, and others, like Elizabeth in particular, get name-dropped as the battle begins. Fans may find it a nice callback to the older series.
The first blow from Galand threw Gawain and Percival through several buildings. Gawain certainly proved her strength as she tanked and dodged Galand's blows, while Tristan broke out of Melascula's trap. Teamwork ended Galand, as Gawain sliced him in two, followed by Percival's Hope Clone inside Galand detonating and creating a massive crater in the middle of Liones. An inner detonation seemed to do the trick, reducing the reborn demon back to a helmet.
Lancelot, meanwhile, didn't seem to be helping anyone at first. When Prince Tristan was having trouble with Melascula, however, he assumed the form of a small fairy and triggered Melascula's brainwashing to go into overdrive merely by mentioning what her true self was supposed to be - "Faith" Melascula of the Ten Commandments and servant of the Demon King.
This caused her to assume her true form, a colossal serpent that threatened to consume everyone and everything. It also made her an easier target for Prince Tristan's dark magic, since she summoned a demon that absorbed light and dark magic that was dealt with, thanks to Gawain and Tristan. Melascula met her end at Tristan's hands, cursing Tristan for being exactly like his father.
Final thoughts
The sheer destructive force of the two reborn commandments was laid low by the Four Knights working together. The majority of Four Knights of the Apocalypse episode 21 was dedicated to this fight between the two opposing forces. It's not that much more complicated, aside for the brainwashing that Arthur's company put the Commandments under.
Four Knights of the Apocalypse episode 21 shows the titular Four Knights' power better than episode 20, but also highlights how they still have a while to go before trusting each other fully and acting as in sync as Percival's group has throughout the series thus far.
Aside from the majority of the episode being a fight scene, one of the other things Four Knights of the Apocalypse episode 21 shows is the villains not all out attacking, but planning and carefully diverting attention with bigger and louder threats while they sneak in to achieve their goal. It's typical of greater villains to sacrifice their pawns.