This achievement is unlocked by reaching level 30 with the hunter class.
As you start off your adventure in PSO2, first get some story missions done to learn the ins and outs of the game. Then take on entry level quests to gather weapons and armor. As you approach the teens, take on bigger challenges if you feel you can handle them. If you need assistance, join a party via quest matchmaking. Most people will let you join them.
Do as many daily orders as you can. The exp they provide will help on the road to level 30.
If you see an urgent mission banner appear at the top of the screen, get ready to drop whatever you're doing, cause these events are your ticket to leveling up quickly. Huge crowds of enemies, boss battles, absolute chaos; it's a sight to behold. This is also where you should use the majority of your exp boosts. And when you get access to it, bump up the difficulty to hard for better rewards and more exp.
As you get into a rhythm, the grind will become more tolerable. Eventually, you will be able to level up a subclass (Fighter, etc.) while playing your main class, which will make getting all the class achievements easier.
Finally, if the achievement doesn't unlock when you hit level 30, switch your character's class, then go back to hunter. It will pop afterwards. This can be done by taking to the guy at the class counter in the gate area.