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 'Dimensional Decimator' device. This device is thrown down and it sucks up anything in its path , just throw it into their spawn and that should grant you a lot of kills. If you have loads of shards/money you could boy yourself a even bigger Xp boost li ke a 5x or even a 6x . 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 :) And after activating Prime mode I ranked up even faster.
My favourite weapons which I liked to use which I found where the easiest to keep the enemy in their spawn and stop randoms from stealing your XP where the Riot Cannon and the Chaos-Rift Combustor. Because with these weapons fully upgraded you'll be unstoppable :D And don't forget when you rank up and do challenges you get gear boxses which you can either open to get them weapons and upgrades or you can save them for when you hit Prime mode. There are five different Prime modes , they go in order as Iron prime , bronze prime , silver prime , gold prime and then Primus mode which is the max level which will make your icon in escalation turquoise , after you hit Primus you will have the full arsenal of weapons and characters at your disposal. That's if you wanna carry on ranking up :)
I also recommend using the Ammo supply core ability because that will save you from having to move to get ammo you can just resupply off your beacon and if your team mates get into it and resupply that will put you a step closer to another achievement where you need to get your team mates to resupply off your beacon a certain amount of times.
And I recommend using the Quickshots Hack , it makes your gun fire extremely faster an give you a +150% XP boost , also good if you have the riot cannon , it will shred through enemy's like nothing :)
My Character setup goes like this:
Primary Weapon-Riot Cannon- Fully Upgraded
Heavy Weapon-Chaos-Rift Combustor-Fully Upgraded
T.E.C.H.-Energon Recharger , Battle Drone , Dimensional Decimator
Ability- Ammo Supply Core
Obviously you don't have to use that , I mean everyone has their own play style but that's just mine in case you didn't know what to use , iv tried everything in the game and think these are my favourite. I hope this helped a few gamers out and good luck to all you getting the achievement :)