Spot Assists are easy but they mean you'll may need to take a back seat while playing. They can only be gained via team type games though, so not in a FFA. This is pretty much the same as the campaign, so go into your Visor () and scout the map for the other team players. Once you've spotted one, hover over them and they'll be tagged.
Now that they're tagged, they will pop up on your teammates HUD. Now all you need to do is hopefully wait for them to kill the guy you tagged for you to get Spot Assist EXP. Preform 29 more and the achievement will unlock. These cannot be gained by playing in a Private Match.