The spined devil, being just cr4, appears too weak to sufficiently challenge the party (it will probably fall in 1 hit, and cannot hope to contribute meaningfully to combat).
The players will be on a small open area, with a small house, in the middle of a ancient forest.
They will interact to a mute man (spined devil) and his small dog (nessian warhound). The man will call his "wife" (Rakshasa) and pcs will be forced to attack her (by a powerful magical mean), but she will attack them too, so it's not gonna be a surprise...
...except that the man an his dog changes back to their devil appearance, female rakshasa not. Dog will be tanking, spined devil will fly and ranged attack and the rakshasa will spend her first round climbing to small house's roof then start casting spells.
There's a reason these two devils are there with her, so I'd like to keep them, although hellcats and imps could do it better...
I think you are underestimating the spined devils... These little buggers have a VERY high AC for their CR coming in at 24 with a touch AC of 18. Their ranged attacks have a +11 bonues and deal 1d4+1d3 damage.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.