Completing 100 matched games means ANY game mode that involves other human players--Coop or PVP.
Not Recommended but faster: The absolutely fastest method is starting up a custom pvp with a boosting buddy, and one person quits out. The quitter gets no XP, but it's way faster than actually playing against AI in Coop! Watch the achievement tracker to make sure it counts depending on when you quit out though. I don't recommend this though because you'll have to play more coop games anyway to get to level 30--you don't get much XP for custom matches.
Recommended: The fastest coop method I've found used Ultimate Jackals, Ultimate Samson, Ultimate Fuel, and the map Twin Peaks. Spam Ultimate Jackals until the timer runs out, then sit and watch the fun! Just over a minute per match.
If you find randoms, this will work as well, although a lot of randoms quit as soon as you start spamming Jackal's or Buck's. LOL. Doesn't matter, you'll win and it counts. Some randoms love it though and will race you for the kill.
Those Ultimates will cost a bunch of Diamonds, around 4500-6000, so you might want to settle for the regular versions until you have enough Diamonds to buy them, or you could buy the $9.99 Starter Pack which I recommend--the games free after all, and it's a lot of fun! You can get more Diamonds by completing quests, or possibly selling items on the player market.