Hybrid

Before you search for a match you'll be asked to choose from one of three possible missions in order to earn bonus exp. The easier ones include a particular number of kills or kills with a certain weapon. If you manage to complete this task in the game, this achievement will be yours after you've done this in 50 matches.

It's worth noting that if you don't unlock any gear your options for challenges will be much smaller. This means you'll most likely have more generic kills/headshots/basic weapon challenges. These vary depending on where you're searching on the map, so if you're unhappy simply pick a new area.

Just a quick note, this does not need to be 50 unique missions or anything. Just keep picking ones you realistically think you can achieve and you'll have this in no time. Simple and straightforward.