Mindjack

A nice way to get XP without really trying too much is to go to level 10-2, youll see two rocket guys on a balcony and other guys running up the stairs. Clear the stairs and kill only 1 rocket guy. Run to the bottom of the stairs and the rocket guy cannot see or shoot you.
Now that you havent killed him, the game keeps spamming guys to you through the gate at the bottom of the stairs, keep getting a headshot on each guy to get 30xp each, keep going for as long as you like. once you feel like you have enough, run up the stairs and kill the rocket guy. :)

As noted by B8TINGU youll need around 66000xp to get to level 50

Alternatively you could complete the campaign then play chapter 12 with another person but be on the red team. Take over Lyle and shoot Rebecca first, then shoot Jim after. This gives you about 450 each time and a win as the red team towards your 50 wins.

The other solutions offered do give fairly quick points, but there is a technique that may be faster. This method assumes you have finished the main plot and that you have a boosting partner. It works best if both players have installed the game to their hard drives to decrease load time.

Under Scene Selection from the Host Portal, set up a game to start on level 12-0, Echoes. Have your boosting partner hack into your game as a Red Hacker. As soon as both players are in the game, press Start to skip the lengthy cutscene.

The boosting partner then immediately presses RB and then RT to take over the bad guy (name omitted to avoid spoilers). You simply walk forward and pick up the shotgun, then walk around to the left and shoot the bad guy in the head. It may take two shots, but not many. This nets a quick 960XP and then the game saves. Once the loading screen is finished, push Start twice, once to skip the cutscene and a second time to get to the menu, where you select PORTAL MENU. Repeat until you hear the sweet ping of success. Mine popped when I had 65,776 XP.

Though hardly the point, you will also be working toward

Good luck to you.

If you're doing this solo and have completed the game you can choose to continue the story. This takes you to the checkpoint just before the very final boss. Defeating him nets you ~1800 XP each time (or the equivalent to 60 soldiers) so you can boost your level quite quickly.

For this way to get to level 50 you need a friend.

The person who wants to get lvl50 needs to be the hacker and get in to the red team. you hack into a npc and kill your friend with melee attack this way you get between 200 and 300xp each time and the npc from bleu team will keep your friend alive. the only thing is that the blue npc may not see you otherwise she will kill you. i did this in the begin of the LVL Panic Room in the small passage before the first stairway and killed 2 of the 3 red team npc's.

I hope this will help if you don't understand and got a friend that wants to boost i'm willing to show how its done. cause it's for me the 1st time to make this.