I actually have seen Luma just eat 3 jabs randomly and take zero knockback.
That's odd. Anyway, I'm going to update my Rosalina matchup tips. I find she's important to learn since she's one of the most popular characters in the game with more than several noteworthy players.
Here's some tips against Rosalina. I played my fourth set against Neos at CEO and here's what I learned:
-Stay away from Smashville. You risk the Luma surviving on the platform when it should be gone.
-Do not airdodge if you get hit with Uair. They WILL read you and punish you hard
-AETHER. Get to the edge as soon as possible and snap the ledge. Using Aether will do three things
--Send you down as quickly as possible
--Activate armor
--Possibly punish Rosalina since the blade starts below Ike, damaging her greatly
-Remember not to abuse Aether as she will instead just position herself directly below you to setup another juggle. Use sparingly.
-Jabs are her biggest enemy. It's a very reliable way to get rid of the Luma safely.
-Dash attack also does the job nicely on an idle Luma, as long as you're far enough away from Rosalina.
-Using jabs is also preferred to hitting her with aerials. She will beat you in the air. On the ground, she's pretty much limited to jabs (only effective with Luma), dashgrab, and dash attack (the dolphin-hump that hits twice). Jab is a great tool for getting her offstage and punishing.
Rosalina loses momentum the moment she is offstage. While recovering, she is limited to:
--Jumping up for a Fair. It has a disjoint and she'll use this at the edge while trying to jump back onstage, usually from below you
--Uair below the stage to "shark" you. This is also to disrupt your Eruption
--Recover early. Up B either onto a platform or behind you to throw off Eruption.
-Getting on the ledge, not to ledge snap, will throw her off completely. Given her large hitbox and floatiness, they may retreat or save their jump. Either way, you can plan an attack from there.
If you're off stage while trying to recover...
-She will use Side B to spam Star Bits. Just hang in there and take the damage. Remember, you die much later than she does, so tough it out and screw the timing up to throw her off.
-Don't be predictable with your Side B or Aether, and don't sink too low. You don't want to get hit with a Dair spike and die early.
Most effective ways to kill Rosalina...
-UTilt. I cannot stress this enough. Rosalinas are culprits for being the most abusing dash-grabbers next to Captain Falcon, Sonic, and Olimar players. If she attempts this and you spot-dodge in time, Utilt will take her out around 80%. The best time to use this is when recovering from the ledge (since if you get up neutral, you're still susceptible to grab). You can also do this while cornering her. She may use dash-grab as a mixup if she has no Luma.
-Dair spike. Given her recovery style and floatiness, if she goes low, Dair is effective. I used it to finish Neos at 40% while I was at 150% or so.
-Attrition. Keep whaling on her till 130% or so and playing safe with jabs and whatnot. Corner jab combo or DThrow.
Remember, Rosalina LOVES to abuse dash grab and dash attack mixups. Their aerials have huge hitboxes and they want you in the air to juggle you with Uair. Stay on the ground as much as you can, grab and don't pummel if a Luma is present. Rosalina is one of those exception characters that you're better off not trying to Fair/Bair to kill as much. She's a special case where you have to change it up since her aerials are effective enough to be ours out. Grounded defense is emphasized.
Edit: Forgot to mention one more VERY important thing.
When you're in the corner, Rosalina LOVES to stalk you with oddly timed jabs. This is to "jab-walk" the Luma close to you before holding A for an infinite spin. This is not only extremely difficult, if not impossible to avoid if you're already on the corner (as in, you can't roll out of it), but at high damage, the last jab hit can kill. She can also pincer combo you if you happen to roll towards her and away from the Luma, where you'll get hit by her wand jab instead.