So far I have only done the online in this game but here are a couple of ways to obtain titles:
Get characters ranked up to level 10: You get one title for each character when you hit this mark. There are 28 characters out of the box and obviously more as DLC so this is a good chunk of the titles earned if you grind this out. I believe the gauntlet mode is a great way to grind the character levels out as it seems to be the preferred way to also rank up to get treasures and your party members also level up as well so it's a win/win situation.
Play 300 matches online with each different summon: The first one I unlocked was Shiva as she is really popular online but this was the requirement.
If I find more ways to unlock titles, I will update here.