What spells can be used to damage a Rakshasa before L7 slots are available?

Question

I've been looking for the most effective ways to damage a Rakshasa as a spellcaster, before L7 slots become available. After reading the 3 questions below,

1. Is a Rakshasa immune to Animate Objects?
2. Can Maximilian's Earthen Grasp or Bigby's Hand target a Rakshasa?
3. Does a Mundane Arrow Fired From a Magical Bow Count as a Magical Weapon Attack?

I'm wondering if the following spells will work:

1. Magic Weapon: Per the last question, my reading is that a Wizard could cast this on a crossbow (or bow, if they have proficiency), and deal semi-decent damage (assuming they are good aligned, it would target the R's vulnerability).
2. Alter Self (Natural Weapons, Piercing): Similar idea, available to other classes, though would require going into melee (not so problematic due to damage, but due to Curse).
3. Elemental Weapon: Similar idea, though I'm not sure if the bonus elemental damage applies.
4. Shillelagh: Doesn't target the R's vulnerability, but a Druid could go this route if desperate for damage.

Are there other (better?) ways for casters to damage this sucker before L7 slots are available?

Note: I'm aware that buffing/assisting is another way to contribute to the fight--I'm interested specifically in offensive options available.

Answer 1

Other than summoning spells, you don't have a lot of options

The rakshasa will be immune to damage from all of the spells you mentioned above. Jeremy Crawford has addressed several of these spells:

Q: Hi Jeremy, does a Rakshasa's 'Limited Magic Immunity' protect it from weapons enhanced by shillelagh or the magic weapon spell since they are spell effects?

A: Shillelagh and magic weapon are spells. Limited Magic Immunity prevents a rakshasa from being affected by spells of 6th level or lower, unless it wishes to be.

As elemental weapon is a similar weapon enhancing spell, I have every reason to believe that the ruling would be similar on it.

As far as Alter Self goes, the damage you could do (and the fact that the damage is magical) are parts of the effects of the spell. And:

The rakshasa can’t be affected or detected by spells of 6th level or lower unless it wishes to be. (MM, p. 257 post errata)

The fact that summoned creatures can still damage the Rakshasa was also explicitly spelled out by Jeremy Crawford and justified thusly:

A rakshasa's Limited Magic Immunity offers no protection against a summoned creature. The creature's presence is a spell effect. The creature itself is not a spell effect.

In all of the spells you mentioned, the damage done is itself a spell effect. So the Rakshasa will be immune to these effects.

Since the Rakshasa has immunity to "bludgeoning, piercing, and slashing from non-magical attacks", one of the only ways to injure a Rakshasa with a spell of 6th level (or lower) when it does not wish to be damaged is by summoning a creature that can do some other form of damage. An example would be casting Conjure Elemental (a 5th level spell) and summoning a Fire Elemental. This Elemental's attacks would do fire damage, to which the Rakshasa is not immune.

Answer 2

Magical Effects that Aren’t Spells

Are there other (better?) ways for casters to damage this sucker before L7 slots are available?

I actually had a Paladin in such a predicament recently.

A Paladin’s Divine Smite power is a way to expend spell slots to damage a foe, but it is not a spell. Thus it can damage a Rakshasa.

Mike Mearls tweeted to this effect about Divine Smite and Rakshasas.

(This is the only power that comes to mind that would work like this.)

Spells the Help Indirectly

As others have mentions lower-level spells can only cause a Rakshasa harm indirectly.

You could charm or dominate a creatue and get it to attack the Rakshasa.

Another spell that could be useful is Enhance Ability: Bull’s Strength. This could give a character advantage on grappling — Strength (Athletics) — checks, which could conceivably help the character force the Rakshasa into a trap or over a ledge.