Teridal/Seika

To unlock Bard, you have to acquire 75,000 Dungeon Points through PvE events such as Dungeon Arenas, although you can scrape points by defeating dungeon bosses. After you get the right amount of points, speak to a Dungeon Master, there is one in every town and you'll find the 'Bard Charter' in the shop.

Unlocking Chronokeeper and Dreadnaught is A LOT more difficult as you need to PvP in order to get points which for the two classes is 75k Battle Points each; the best way to acquire PvP points is through Ahadile Pinnacle which pits two teams in an arena style setting. Once you've got the points, speak to a Battle Master, there's one in every town and there, you can purchase the charter.
Since the player count has been extremely low for the last half of the year, getting games of Pinnacle is hard. But there are other ways although very slow ways of getting BP points, such as taking castles or through regular player killing.