Plane Shift can be used on an unwilling creature, the rakshasa would need to hit with a melee spell "attack" and then the unwilling creature gets to make a Charisma saving throw. I gave the example of targeting the cleric since it would not be a primary stat and Clerics are probably a big threat to a rakshasa.
Using it on an unwilling creature is on the third part of the spell here on DnD Beyond.

That was an example for how they could fight of course, a rakshasa would most likely use it on itself to escape a bad situation considering how they are supposed to be smart and have escape plans. 

Oh, wow. I didn't notice that sorry! I should have thoroughly read the spell before saying anything, thanks for clearing that up!

That really does make things interesting...

Question: If a player casts Zone of Truth on the disguised Rakshasa, it doesn't affect it because of Limited Magic Immunity. However, does the Rakshasa still roll the saving throw?

Quote from zevhur >>

Question: If a player casts Zone of Truth on the disguised Rakshasa, it doesn't affect it because of Limited Magic Immunity. However, does the Rakshasa still roll the saving throw?

 No, but the DM may still roll a die just to keep up the charade.

Quote from Ookimow >>

Quote from zevhur >>

Question: If a player casts Zone of Truth on the disguised Rakshasa, it doesn't affect it because of Limited Magic Immunity. However, does the Rakshasa still roll the saving throw?

 No, but the DM may still roll a die just to keep up the charade.

 This is good.

 I hadn’t thought of that.

Oh, the Rakshasa doesn’t have the “self only” restriction on its plane shift, like most Innate Casters.

It can grant others passage between Planes, without being limited by the need for a Tuning Fork.

That’s a big bargaining chip.

I would think True Seeing would in a sense not be an issue as the spell is cast on the caster, not the rakshasa, and grants the caster 'True Sight' which is an ability without any spell level attached.  The Rakshasa by example would still have it's true form/nature revealed to any creature with that ability as a special sense (e.g. Pit Fiend) in the same manner as Tremor Sense or Blind Sight would locate it

I'm going to chime in further that the Rakshasa needs some good area effect type spells of 6th level or lower it can just cast and wade through while combating the party.  Fighting the adventurer's in a Mansion Protected by 'Guards and Wards' while concentrating on 'Fog Cloud' or 'Cloudkill' (or both with 2 rakshasa's) could dastically increase the danger it presents.  Although, I don't feel it earns it's challenge rating in a number of other ways, including the damage dealt, I think this could increase the threat a party would feel, particularly with minions present who might be immune to poison, webs, fire, or difficult terrain.  

Edit: I also feel like a number of sneak attack dice added to the creature would be thematically appropriate and fit it's overall design.  Poison, maybe on a magical blade, because I think back to my old school days of BG2 and the similar wielding rakshasa's there.

I realize it's been almost 2 years since you posted this, and hopefully you have figured this one out, but for the sake of other new DMs out there here's the reasoning for the high CR.

The bigger resistance that you have to contend with here is the "Limited Magical Immunity". The text is a little vague (typical of DND5E) but can be taken literally. Rakshasa "can't be affected or detected by spells lower than 6th level unless it wishes to be." Further it has advantage on all saves vs spells / magical effects.  Straight up immune to magic - Magic Missile? Acid Arrow? Fire Ball? Disintegrate? or any spell cast by a warlock (not including their advanced mystic arcanum spells) the Rakshasa can simply ignore and take no damage. Further it's a masterful disguise artist with it's Disguise Self and invisibility spells so you won't realize  you have a rakshasa on your hands most likely until much later - oh and the immunity says it can't be detected so your caster has to be able to cast at least a 7th level spell that allows you to see through illusions/invisibilty to detect it. It will also 100% have minions under is domination or suggestion (or just paid very well, as they tend to hoard wealth). 

It's not much of a physical threat - relatively low damage for the level you would probably expect to encounter them - and they have limited hit points even with it's immunity to non-magical weapons. But it will have minions or possibly dominate a party member - and if your wizard/sorcerer/warlock etc can't hurt it with their spells, and it dominates them, those spells will be turned on the party for sure. 

All of this said - CR is basically a guideline, and you could probably throw this at a much lower level party, and they'd be ....well challenged :)  Given some good hints and time to prep it could be handled as early as probably 8th or 9th level. 

The magical immunity shuts out wizards at lower levels, and it's worth noting that Raksasha's use their intel gathering and charming abilities to accrue lots of minions.

Divine Sense is not a spell, nor is divine smite.

 its one of those monsters that depend on the skill of the DM. No offense to you, but alot of dms gave a habit of making monsters into Clone Troopers

Well Its immune to spells sixth level or lower, and that refers to the level the spell actually is, as in the level you unlock it at so at that point you as a Dm are just babysiting the Pcs, but at that point if it makes it spell save it would tke a quarter damage because it already has resistance to magic damage 

Immunity to all spells of 6th level or higher is really op especially if you are playing a wizard. Can they still be damaged by any spells that are not 7th level or higher ( like maybe a cantrip ) because if not that ability makes them way overpowered

Is there a way on D&D Beyond to set the Rakshasa as your race for a character?

Edited: you can find multiple instances of the race in the homebrew section... but why you would want to play a so overpowered creature (should you remain on the physical plane) is beyond me ;)

Having read other sources that go more into depth, you nailed the reason for the high CR: Setup and intelligence. 

As a powerful, nearly immortal, creature of the Nine Hells they are likely to come by powerful magical items and have minions of their own. Sadly this entry says nothing about how they prefer to avoid direct combat, especially in melee, or anything else that makes them so dangerous. Good luck finding a shapeshifter crime lord in a city without divination magic.

Quote from DEMaS >>

So, common consensus, would a counterspell cast lower than sixth level be automatically resisted? Or would a player get a chance to roll? I am thinking that it would be resisted through the ability without any opportunity to be countered, but 5th is still new for me.

 No because counterspell targets the magical energy used for their spell rather than affecting the Rakshasa.

A rakshasa is not a monster for a strictly combat scenario. The skillset of a Rahshasa is one of an enemy that lurks in the shadows, dominates NPCs, and manipulates people around your players to create conflicts and interesting plot lines. A rakshasa does not like to fight. He will usually plane shift away if stuck in a corner. That is if he decides to plane shift himself and not one of your players.  A Rakshasa is a good recurring villain and behind the scenes puppet master to the events that you unfold. Its absolute immunity to spells under sixth level makes any caster under 11th level basically useless, and it can just fly away from or dominate the martial characters. Even in combat, it can be a beast to defeat. I've ran one before and he was a true adversary to my party. 

*Whispers* Rakshasa *Jazz hands*