Defeat the Ice Queen -
In order to fight the Ice Queen you must answer Toya with “NEVER” when he asks you if you are ready to kill him. Toya is not very hard but if you use your EX moves to beat him you will need to raise your EX all the way back up to defeat the Ice Queen. There is a VP recovery and a store right after you fight Toya so use this to restore your VP and go back to the main chamber of the Ice Castle to raise your EX back up. Before continuing up the stairs to the battle buy all the healing items you can and spend the rest of your TB on SP gems to boost your HP.
I used all 4 human characters at level 60 with direct, support and agility all maxed at 999. I also had all the moon weapons which are obtained in the Holy Beast Shrine. Each character had over 3k health. I used the EP+20%, Cut EP use, HP+20%, and the free movement supports on all characters. At this point I had done everything else in the game minus grinding out leveling and SP to boost my HP and EP. Even with my characters maxed out like this the Ice Queen was still a formidable opponent. She was even harder than the Omega Golem.
The Ice Queen has 2 forms and I found that the first form seemed extremely harder than the second. I always started with Raigar casting his 50% defense spell on everyone and Atsuma using high charge to boost his PRM by 50%. Karin was used for healing and Yuki was used for damage to The Ice Queen’s allies. I stated earlier that you need to have your EX maxed and this is because it makes the battle much easier. I died about 10 times trying to defeat her without raising my EX before the battle. Use Atsuma’s EX ability Stun Bead Fire to attack the Ice Queen. It took 2 strikes of Stun Bead Fire and a few random attacks from other party members to take her out quickly.
Once the Ice Queen has changed to her second form move Atsuma, Yuki, and Karin to the very back row with Karin in the middle. Place Raigar in front of Karin. Use Atuma’s High Charge, Karin’s Healing Spell, Yuki’s Mega Shot and Raigars 50% damage decrease spell. Once your team is in this set up do not move them just spam the Ice Queen with whatever attacks can hit her (Mega Shot, Flare Wave, etc.) from this distance and heal with Karin as needed.