A good way to beat La Saia is to first have a high attack level on all your weapons, and collect a lot of fire stonesto throw at her. Finally, set everyone's weapons on the Fire Attribute, and bring along Stanima Drinks, Premium Chickens, and Weapon Repair Powder.
You have to use Toan or Goro to deal the initial damage. If one of them gets frozen, switch to the other character (sometimes switching back and forth will shatter the ice around the frozen one), and try it again.
Once the damage is done, switch to Ruby or Xiao and hide behind one of the pillars. Use Ruby to do the first damage, and then Xiao until 'No Effect' shows up. Repeat all of this--use Toan or Goro first, and then Ruby or Xiao to finish her off.
This is the strategy I use, but I like a little edge, and already had leveled up to Toan's Arise Mardan (reward sword from fishing), Chronicle, etc. Also had Xiao's Angel Gear and Ruby's Athena's Armlet. Having a little edge never hurts. ;^) Hope this helps.
