As stated, the amount of XP you need for each class is 362,500. The easiest way to do this is through Conquest.
You can either do this the normal way, or you can boost. If you want to boost, the easiest way is going to be similar to the Gears boosting, except you don't get to sit around for 10 minutes as in Gears.
One method is to get in a large party with the understanding that no one kills another, and just run around capturing nodes as fast as possible. That's because each person gets 150 XP/node, while only 50 XP (plus possible bonuses) per kill. If the nodes flip over fast enough, then you'll get more points focusing on taking nodes, which will be faster than spending time trying to kill someone.
The other method is to get everyone at one node and keep taking and retaking that node. Each group takes a turn, which are going to be fast. The more people on a node, the fast the node converts, so it's ideal to have 5 people/side. Everyone meets at a particular node, A, B or C, it doesn't matter.
One group captures the node, backs out quickly, the next group rushes in, takes the node, backs out as well. Over and over, 150 XP/turn... you get the idea. Only play one node because the round will take longer (meaning more XP/game) and it'll be easy to quickly take and retake that node. Like I said, it works best if all 10 players can rotate as fast as possible.
Either that... or just play a lot of games.