Disclaimer: The Game has change so much the first time they release it and this info might be wrong now see the last comments for details
To complete this achievement you need to max out all the class:
Assault, Recon, Sniper, Heavy and Support.
Each class require a total of 60 skillpoints that's 300 skillpoints in total.
Skillpoints can be earned by leveling up or doing events and event squad.
Max level is 120 that's 120 skillpoints. You get 2 SP for every level (except levels 10, 20, 30 and so on) with the recent update. So every 10 level up you are looking at 18 SP. Posted by ArDPalang
Other tips to max out a class is to buy the Bundle and it's located on Black Market
Advance Assault 400 credits, Advance Recon 400 credits, Advance Sniper 600 credits, Advance Heavy 1000 credits, Advance Support 2000 credits.
Update: From "CroutonCrusader" I just unlocked the achievement and just wanted to confirm that the new class *is* required to unlock the achievement.
I suggest spending your skillpoints on the Support class that will unlock at level 50 then move to Heavy, Sniper and earn credits on events to buy the Assault and Recon for 800 credits.
Hope this helps leave a comment if I miss something and good luck to you all