Make a Wish, Aarantula

Badges: Triple Dip, D-Down Jump, Jump Charge
Items: Shooting Star x8, Repel Gel, Kooky Cookie
HP: 20, FP: 15, BP: 6 = Level 7 (Take at least one Star-Boosted attack per phase)
Items: Shooting Star x8, Repel Gel
T1: Star Storm (7), Shell Shot (12)
T2: Star Storm (19), Shell Shot (24)
Star Shield.
T3: Smooch, switch to Goombario
T4: Tattle, Star Beam (dink)
T1: Bow, Jump Charge
HP: 1/5 (5%/25%), FP: 14
T2: Peach Beam, Outta Sight
FP: 12
T3: Triple Dip Repel Gel + Shooting Star x2 (36)
FP: 6
T4: Watt, Shooting Star x3 (54)
FP: 0
If Star Shield is used this turn...
T5: Kooky Cookie (Invisibility), Turbo Charge
T6: Peach Beam, Electro Dash (59)
T7: Triple Dip Shooting Star x3 (77), Electro Dash (82)
T8: D-Down Jump (94), Electro Dash (99, KO)
Else (T4)
T5: Electro Dash (59 damage). Kooky Cookie (Invisibility)
I'm pretty sure Star Shield gets used this turn.
T6: Peach Beam, Electro Dash (64)
T7: Turbo Charge (1 turn), D-Down Jump (76)
T8: Triple Dip Shooting Star x3 (94), Electro Dash (99, KO)
Okay, so I really only have a rough idea of how Bowser's AI works. If Mario's HP is 5/20 or less, Bowser's AI won't ever check to heal himself and I brought a couple of items I need for the sole purpose of not getting hit. If there's other turns Bowser could have used Star Shield where I didn't account for them, I legitimately cannot parse his AI to find out when.
Even if I did catch all of them though, this fight is far from guaranteed. I ended up bringing a Kooky Cookie with me because I needed to be invisible for 3 turns, not 2. So, a second Repel Gel was just not going to cut it. I also just didn't have the space for a third Repel Gel so I essentially had to just make a wish and pray that the Kooky Cookie granted it.
I got it on my first try.
So uh... this strategy is a mere 40% success rate. Make of that what you will. I didn't bring Shooting Stars with me into Chapter 8, but the shop sells them so I was set to make this work. I'd wish I could have come up with something better, but A.) I actually really like how this fight came out and wishing for something else feels like looking a gift horse in the mouth, and B.) I didn't have any time left to come up with anything better.
Also something about it being thematically appropriate using mostly Shooting Stars against a boss we fight in space using wishes against us.