I struggled with Mattias even at Lv. 60 until I read an easy way to cheese him with the Reflective Veil skill. Bring two clerics, and just stack veils on your entire party (you can also use Dancer Divine Skill so that it gives it to everyone from just one cast). Slowly take down his HP using basic attacks from your other two party members that are NOT strong against him. All you have to do is keep your veils up while avoiding breaking his shields. Basically if you never break his shield, he will never spawn adds. He will keep spamming elemental attacks (that will damage himself) so all you need to do is be patient, keep your HP and veils up, and you will take him down in no time.