Peerless; unrivaled. Used to indicate something or someone is exceptionally excellent or unmatched.
彼の剣の腕前は無双だと評された。
His swordsmanship was said to be peerless.
その作品は技巧・感性ともに無双の出来栄えだ。
That work is unmatched in both technique and sensibility.
歴史上、無双の武将と呼ばれる人物が何人かいる。
There are several historical generals who have been called peerless.
Slang (especially in games): mowing down the enemy; dominating and defeating many enemies single-handedly.
あのボス戦で彼は無双して全滅させた。
In that boss fight he literally mowed them down and wiped them out.
新しい武器を手に入れてから、彼女はマルチで無双状態だ。
Since she got the new weapon, she's been dominating in multiplayer.