When you start off the MP you want to go for Escalation and just play as what you would prefer (Autobot or Decepticon) and once you find a match or you end up as host vote for 'Obsidian' because I found this the easiest map for xp because you can spawn trap the enemies in one of the spawns , if you want to find this spot , once the map loads up just keep going straight until you look right and you see like a red wall which the enemies come out of , there you can spawn trap them , but only for a short while because bigger enemies start to show up like the leapers at like wave 5 or 6 and the titan at wave 11. There are also these little blue boxes that spawn each time you finish a wave , you want to get them because that will also help you gain xp.
There is also a cool trick with the boxes because when you play online they only grant you 100 points and the bigger blue ones give you 500 but if you set up a private match on escalation on hard mode and go into it the boxes should be worth more , blue boxes 125 , the bigger boxes 625 , but you don't have to play on hard you just back out after the map loads up then search for a public match and try get in a match where you are the host and it should grant you the same xp as it does on hard mode even though you are playing on the standard difficulty , just every time you see boxes get them! And try play as a flying character because you can manoeuvre round the map easier , bigger boxes don't spawn till waves 6 and wave 11 , some are underground and the other box is on top of the roof where you going to be spawn trapping. Also up on that roof right at the back is a sort of ability box which you can buy things like 2x-6x XP boosts to level up faster and there is also a ability on there which is worth 1000 shards and is like a black hole device not sure what it is called but if you chuck one of them done in the enemy spawn it will pretty much kill everything.
my method for when I was leveling up was to stay up on the roof of the building where the ability box is and just kill everything till there is one enemy left and just go around getting all the boxes then kill the last guy. Then once the wave is on the countdown I go buy a 3x XP and a black hole device. A good way to keep the leapers off your back is to buy a sentry turret and that should sort them out. If you come face to face with a leaper just try dodge him and shoot his back. When you level up and do challenges you get gear boxes and they will grant you items like ability's and new characters , also new weapons which can come in handy seeming the weapon you start with is junk when you get to higher rounds.
I hope this helped in some way and I hope it made scene , once you hit Rank 25 just back out and go into Prime mode. It is pretty easy to get to Rank 25 , I once I started playing I was level 25 in like 2 hours :)