Here is the full list of all 20 Kaiju Princess achievements.
Obtain the Bikini
Obtain the Magical Teen Outfit
Obtain Black Cat Cape Doll
Obtain the Maid Outfit
Reach Mouth Accomplishment Lv.3
Obtain Tilapia Doll
Obtain the Competition Swimsuit
Obtain Festival Fan Doll
Reach Breast Accomplishment Lv.3
Reach Clit Accomplishment Lv.3
Reach Vagina Accomplishment Lv.3
Reach Orgasm x8
Reach Orgasm x4
Kisses x1000
Obtain the Witch Outfit
Obtain Popsicle Kaiju Doll
First Quota Reached
Reach Orgasm x2
Cum with Me
First Time