Roronoa Zoro is the combatant of the Straw Hat Pirates and one of the most important characters in Eiichiro Oda's One Piece. Zoro was the first person to join Luffy's crew after the Straw Hat captain saved him from death in Shell's Town. Being a combatant, Zoro's main role in the crew is to provide his captain with his fighting prowess when needed.
Although he's has been growing powerful ever since setting sail, it was only during the two-year timeskip of One Piece that he was able to tap into his true potential and learn countless new powers under the guidance of Dracule Mihawk.
7 Armament Haki
Perhaps the most important thing that Zoro learned during the two-year timeskip is Armament Haki. This form of Haki allows a person to use their spirit as an invisible armor and increase both their offensive and defensive capabilities.
By forbidding Zoro to drink alcohol until he mastered this skill, Mihawk was able to drill this ability into him fairly easily. Zoro has displayed his usage of Haki in several instances in the story and it's safe to say that his skill with this technique has been nothing short of immense.
6 Armament Imbuement
An application of Armament Haki, Imbuement allows a person to imbue any weapon with Armament Haki to make it stronger. This power was first seen on Amazon Lily where the Kuja were seen storing Haki inside of their arrows to increase their attack potency. Roronoa Zoro learned how to imbue his swords with Armament Haki during the two-year timeskip when he trained under Mihawk.
Thanks to his guidance, Zoro has become incredibly good at this skill. In his fight against Pica, Zoro noted that his swords didn't take even a scratch thanks to how well he used the Imbuement skill. With time, this skill of his has only improved.
5 Observation Haki
Zoro also learned how to use Observation Haki from under Dracule Mihawk, however, this skill wasn't focused on nearly as much as Armament Haki. Zoro, being a swordsman, uses Armament Haki as his primary weapon, which explains why less emphasis was put on this form of Haki. However, his Observation Haki training went quite well, as he's shown the ability to use it efficiently.
On Fishman Island, Zoro was able to sense people from a long-distance away. In Wano, he saved Otsuru's life by blocking an arrow traveling towards her at supersonic speed within a second.
4 Information On Conqueror's Haki
Although Zoro gained great control over two types of Haki during the timeskip, he didn't have the time to learn Conqueror's Haki as he simply hadn't unlocked the ability at the time. Zoro did, however, learn all there is to know about Conqueror's Haki. On Fishman Island, Zoro was able to tell that Luffy used Conqueror's Haki, stating that he'd have to step down as the captain if he couldn't do something like that in the first place.
In the Wano Country arc, Zoro finally awakened his Conqueror's Haki in the fight against Kaido when Luffy's life was on the line. What's more, Zoro even managed to imbue his swords with Conqueror's Haki and managed to scar Kaido permanently, earning Kaido's respect in the process.
3 Improved His Ittoryu
Before the timeskip, Zoro's control over other sword styles was decent, however, it wasn't nearly as good as it is now. It is believed that he trained his other sword styles, such as the Ittoryu, to quite an extent during the two-year timeskip. Zoro was seen slicing a galleon with Ittoryu when he reappeared after the timeskip, which just goes to show how powerful he truly had become.
On Fishman Island, he almost killed Hody Jones with his Ittoryu and even sliced the head of a dragon in Punk Hazard with the very same technique. Evidently, Zoro's Ittoryu has tremendously improved since the two-year timeskip in One Piece.
2 Dead Man's Game
Zoro also worked on his Kyutoryu during the two-year timeskip as this is his strongest style so far. Essentially, Kyutoryu is the same as Santoryu, however, Zoro can multiply his limbs and head by 3 times, making him 3 times as strong as he already is. Before the timeskip, Zoro used this power twice, the first of which came against Kaku of CP9.
He then proceeded to use this technique against a Pacifista on Sabaody Archipelago. After the timeskip, Zoro was seen using a much better technique with the Kyutoryu as he could even scar a Yonko like Kaido with its usage.
1 Gained A Mastery Over Other Blades
Zoro's preferred style of fighting is the Three-Sword Style, also known as the Santoryu. However, during the two-year timeskip, he has massively improved his usage of other styles. Zoro was seen using a seppuku blade in Wano Country with which he managed to slice a building in half, showing great skill. Evidently, this was a callback to Zoro being defeated by Mihawk using a pocket knife on Baratie.
What's more, he also used Killer's, or Kamazo the Manslayer's scythe, as a replacement for his sword by imbuing it with Haki and making himself powerful enough to take down all those who stand in his way.