While this achievement was initially only possible through the use of Plushie Master and Dance, Long-Eared Beasts!, there are many effects in the game that may raise Krilalaris's HP past the initial 6, such as the passive health buff from the Avenger and Dealer role in Co-op Mode, and from the purchasable drinks in Bounty Hunt Mode. Any of these may be use to earn the achievement
This is best done on either a smaller stage with opponents that aren't very interested in bullying you or a stage like Christmas Miracle where battles tiles are optional. Rainbow-Colored Circle and Reverse Attribute Field may be good cards to bring if you're at 8HP and have to battle. This can be done with a single hyper, but if you use more than one in succession the chances of acquiring it skyrocket. Ultimately, there's a lot of chance going on, but it's more than offset by the fairly low requirements; keep playing as Krilalaris, spamming her hyper, and you'll probably get it before too long.