Demons only have a few answers for land raiders:
1) Monstrous creature assault. 2d6 + strength armor pen can get through AV 14. If you expect raiders don't be afraid to update your demon princes with the gift that gives a strength boost.
2) Soul Grinder maw cannon there is an option for a STR 10 shot. This can hurt a LR but note the ballistic skill on a grinder as you'll miss your shot half the time, and even then half your hits will fail to glance/pen.
3) Screamers are jetbike models with melta bombs. They exist solely for destroying vehicles. You get 1 melta bomb attack per screamer but melta bombs can do some nice damage. 8+2d6 armor pen has odds in your favor to get penetrating hits (not great odds, but they are in yer favor).
Screamers also have some cool tactical options as well. As jetbikes they can turbo boost for 3+ cover saves, which is pretty nice. They normally have a 4+ invuln so you only really need to worry about cover to block Line of sight. This turbo boost option is a good control tactic for wrangling vehicles. You can turbo boost in front of raiders to corral them. They either have to go around your unit or try to tank shock it. If they tank shock, you can death or glory a screamer and get 1 automatic hit with a melta bomb, which is scary enuff to ward off players doing that.