Slottm is suggesting a dual-resist team with Odin and Abyss Neptune as leaders. If that is feasible then I have a very good chance of getting a Hera (or two preferably). I am going to farm mech dragons like mad when I have the chance and use those to level up some pieces. If I can get enough (3 would do actually) I will even evolve my Odin.
Tonner - Honestly, maxing out your siren before hera should be top priority. With this team, it's pretty hard to die as long as you play your cards right. A single 3 heart match will keep you alive against hera (or venus really, they're a mimic of each other in terms of difficulty in this dungeon, since hera will have 50% reduction, they both require a manual heal every turn)
Allow me to give you some important tips for tackling this dungeon.
#1 Know your math. Know exactly how much damage you will take in a round. Know your monsters defenses, know their HP. Understand that you will need to manually charge your skills for EVERY round of trash, and be able to keep at least one monster alive in order to charge those same skills back up for the next round. ie; devils will die to a mech dragon nuke, demons wont, nor will mystic knights. Mystic knights die very quickly due to low armor - demons are easy to charge against.
#2 Know that RNG will try to fuck you at every chance it gets. (I can curse on this forum right? never read against it, just checking...let me know if not and I'll edit) Sirens are here to keep you alive. DO NOT WASTE MORE HEART ORBS IN A TURN THAN NECESSARY. Do not waste your siren's skill unless necessary. If you don't need to use your sirens for 50 rounds in a row, consider yourself lucky, not wasteful for bringing them in. They are absolutely necessary and you will get f'd to the point where both cooldowns will be up and you still won't have any hearts on the screen. This is why it's important to be able to take 2 hits - because the next round after taking a full hit, you better heal up ASAP.
#3 You're in it for staying alive - not doing damage. (Against boss rounds) Doing damage is NOT your top priority here - staying alive is. Do not worry about making active combos for damage at every chance. Worry about clearing as much of the board as you can, along with 3 health orbs every turn against her, so that your heart orbs will continue to drop. Damage will come over time.
#4 SAVE YOUR HEALTH ORBS! Have 9 health orbs on the screen, both siren skills down? Use 3 health orbs - do not match anything else. Let your siren skills come back before risking using up all of your utility.
#5 Only use cooldown skills when necessary - you may *really* need them later. Echidna or rainbow keeper being your crutch here. Echidna is a lot more useful because she'll get you out of a bad situation if you're out of health orbs and out of siren cooldowns. Rainbow keeper will just force you to take less damage...which may still kill you. He needs to be used more in advance.