Have your party at least at level 30 (for example, Vaan as party leader, Basch, and Penelo). Have both Vaan and Basch to learn Thundara and equip them both with Ice Shield. Equip Penelo with Storm Spear. Buy at least five Banchus Wine (Berserker). When the battle begins, have Vaan and Basch immediately cast Thundara to kill all the Ice Ace. Then, have Basch keep casting Thundara at Mateus and Vaan heal when needed. Let Penelo keep attacking Mateus. Once its effect wears off, make her berserk again. It is possible to defeat Mateus in about seven minutes by doing this.

Before the battle, equip as many Ice Shields to your party as possible. Get all of your party members to at least level 30. Also, teach Thundra to as many characters as possible. When you are prepared, enter the battle. Immediately get all of the Esper's friends close to her and unleash a chain of Quickenings. If you are lucky they will completely kill her little friends. If not, take out your reserve party (or just use some Ethers and/or Exthers to get back MP) and use your Thundra spell to finish them. The only attack of Mateus' that you really have to worry about is her Blizzaja attack, unless you equipped the Ice Shield, then it will not do too much damage. Also an ability she has may cause Sap, but you can get rid of it using Regen. Keep everyone above the halfway mark and do not cast any spells on her. The Esper has Reflect on herself, meaning that Thundra will be bounced back at the party. Keep attacking and healing to defeat her.