Declan Slater’s hat trick earned Main their fourth successive league win.
Ben Hunter’s side are guaranteed to finish fifth in the Division One table and will meet North Ferriby in the play-off semi-finals.
Rossington have never previously finished higher than seventh in the second tier of the NCEL, which they joined in 1991.
Main finish their regular league season at home to Glasshoughton Welfare on Saturday.
The play-off tie will be a one-legged affair at North Ferriby on Saturday, April 16.
Brigg Town and Harrogate Railway Athletic will meet in the other semi-final.
Hallam have been promoted as champions.