Max level for all classes is 25. This will be the most time consuming achievement as it takes roughly 4-6 hours to get a character fully leveled up (highly dependant on skill). Recommend working on one class at a time. Also, playing Conquest seems like it is the more effective mode in leveling up. Since you get 400pts a capture and about 50pts a kill. So you can really just spend the whole game running from position to position capturing the nodes and racking up a few kills here and there.
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