Ngl I want Cas to defeat Lucifer. I think it would be a way for him to really see that he's not expendable, just a tool, that he's important. Also, he deserves a win.
I think Cas will definitely have a large part to play in Lucifer’s demise but ultimately Lucifer himself is Sam’s plot line - it is too big a catalyst of Sam’s self forgiveness and endgame happiness arc to defeat Lucifer to let it go to Cas, but I feel like ultimately it will be Sam mainly but plus Cas, Jack and probably Mary and probably Dean too who end up playing different roles in god take down, but with Sam as the ultimate protagonist. There’s a reason it was Sam who killed Ramiel the YED and why it was Dean who reconciled Amara with Chuck, these are relevant to their own character arcs.
For me the biggest way Cas will get his hero arc is not by defeating Lucifer himself but through Jack, his positive choices towards him and what will happen now in the future (as I believe Jack will end up being such a positive force in the world, go to @amwritingmeta s page to see her theory about Jack bringing balance to the universe). And obviously this is an irony in itself as Lucifer created him to do the opposite, I also believe Jack will have a big part to play in Lucifers ultimate downfall, again, all the irony.
So yeah I think Cas will get his hero arc through Lucifer, through Jack, but obviously therefore it won’t be in such simple terms as just defeating him, it’s more complex but ultimately more satisfying all round!