Tries from Adam Milner and Jordan Rankin helped Castleford Tigers to victory on the road in a hard-fought elimination final.
London Broncos' brave bid to retain their Super League status came up short, while Salford Red Devils claimed third place in the table.
Jay Pitts' late try means London Broncos remain in with a chance of beating the drop, while also leaving Hull KR in deep trouble.
Four teams are level on 30 points in Super League after Salford Red Devils comfortably saw off Warrington Wolves on Thursday.
Losers in the Challenge Cup final for two of the last three years, Warrington Wolves turned the tables to beat St Helens and claim glory.
Three tries in a dominant spell saw Wigan Warriors ease past potential play-off opponents Warrington Wolves.
Warrington Wolves offered little resistance against Super League leaders St Helens, who ran out 30-12 winners in Thursday's clash.
Catalans Dragons will be without Sam Tomkins and Michael McIlorum for three and two matches respectively following a brawl in Perpignan.
Matty Smith is set to make his Warrington debut against his former club St Helens after joining the Wolves on loan from Catalans Dragons.
Catalans Dragons' victory over Warrington Wolves was overshadowed by events that are now the subject of a Super League and RFL probe.