See my video for campaign boosting for a great way to earn XP. In total there are 65 onyx medals you need to earn in order to unlock the Seriously 3.0 achievement, you will also need to rank up to LV100 but that will most likely come automatically while going for the onyx medals (I'm Rank 59 and have only unlocked 2 onyx medals so far.).
NickHawkeye made a spreadsheet to keep track easier, feel free to use it: Click here. Please shout out if you find an error or if there are is any information you want to provide. Not all of the numbers from the official strategy are confirmed to be true. You do not need to have participated in the beta or be Lv100 in Gears 2 to earn this achievement.
