Heres some assistance for Evangel and everyone else having trouble with Mateus, this is from the Piggyback Game Guide

Mateus is supported by 5 Ice Azers, aim to remove these from the fray immediately. try using a quickening chain, though this is very MP costly. Casting thundara several times is an option but beware if Mateus has reflect activated. Best off to attack them individually with thunder or lightning imbued weapons. Keep Shell active throughout the battle to minimise magick based damage against your team.
Once the Azers have been defeated, harass Mateus with physical attacks, especially if one of your characters is equipped with a Storm Spear. His Blizzaja attack can be devastating, causing damage and inflicting Sap at the same time. He uses it frequently towards the end of the fight. When you see a warning in the Information Box that he is readying this attack, immediately heal your team with a powerful potion or the strongest cure Magick at your disposal. Spreading your team out by having two characters use ranged weapons can be an effective tactic here. Cast Regen on anyone afflicted by Sap, and consider using the Beserk status effect on your primary damage-dealer - especially if he or she wields a Storm Spear. When the battle ends, you will receive Mateus the Corrupt, an Esper.

Hope this helps you out if you are still having trouble. Good luck with the game!