1) Kill 10,000 Snowflakes in Niflheim to unlock ice potion.

2) Choose Hafgrim and go to Niflheim in Hard mode or higher.
- You can choose whatever character you like, but Hafgrim has good DPS and rumoured to be the easiest one to get Ice Hearts. (I got got the Ice Hearts in my second run with Hafgrim).
This needs to be confirmed, so take the character suggestion with grain of salt.

3) Save up some money (recommended at least 100 gold).

4) In store, refresh the selection until you can buy Ice Heart(s).

Keep in mind, that Ice Hearts will go to the last slot of your health bar and thus will take the hits first.
I suggest you unlock Hafgrim's "Unlimited" skill just to get 3 shards that can take 3 hits before damaging your Ice Heart.
Another good character with ability to protect the Ice Hearts is Amaranth. She will have the Green Hearts before the Ice Hearts and you can get a lot of them with "Emerald Bracelet" skill.

Also, if you don't want to jeopardize the Ice Heart before the second store, you can try to risk the change to get it and use all your money in the first store to bulk up your DPS with different items and weapons. Then gather gold after the first store and try to roll your chances in the second store.

Boss fight can be tough if you get more than 3 hits from her ice attacks when playing with Hafgrim.
I highly recommend you set the effect visibility to the second low setting, so it's more easy to dodge the boss attacks which will be easier to see under your own attacks.