Meaning:
“神仙” means “gods and fairies,” and “打架” means “to fight.” First used by gamers, this term refers to the scenario where two or more high-level players are competing in a game. Now it is also used in a more general sense where a competition is attended by the best players or a movie casts many A-list actors and actresses.
Example:
A: 最近有部一战题材的片子很火。
Zuìjìn yǒu bù yīzhàn tícái de piānzi hěn huǒ。
A film themed on World War I has been popular recently.
B: 是《1917》吗？主角是新人，配角倒是神仙打架。
Shì 《yījiǔ yīqī 》ma？Zhǔjué shì xīnrén，pèijué dǎoshì shénxiān dǎjià。
Are you talking about “1917?” The film casts newcomers in the lead, but several A-listers in supporting roles.