I am the Eggman, Aarantula

Jr. Troopa (Level 6, Eggman (only egg items allowed))
Badges: Triple Dip, Double Dip, P-Down, D-Up
Items: Egg Missile x8, Fried Egg
HP: 15, FP: 15
T1: Triple Dip Egg Missile x3 (18), Spiny Flip (21)
HP: 9, FP: 9
T2: Egg Missile (27), Spiny Flip (30)
HP: 2, FP: 9 (assuming the spike attack, guard)
T3: Double Dip Egg Missile (36) + Fried Egg, Spiny Flip (39)
HP: 5, FP: 6 (assuming the spike attack, guard)
T4: Triple Dip Egg Missile x3 (57), Spiny Flip (60, KO)
Jr. Troopa will shift phases when his HP falls to 38 or less and 20 or less.
You can also spend the first 2 turns double dipping Egg Bombs and it wouldn't make any difference.
I used Lakilester because his spiny flip throws what the game calls an egg. So this doubles as Lakilester Single Partner, lol. I just wish I had a reason to use Spiny Surge because it'd look cool for getting around HP triggers for phases.