Part Two : Passively Gaining EXP...
Other then being the Healing or Repairing Class there is a few other ways to get EXP without killing other players...
When in battle, its a GOOD idea to SPAM q...
q (or whatever you rebind it to) is that of Spotting, and as of a recent update during the beta a way to communicate depending on the target.
Spotting a Foe will bring that player up on ALL Ally radar within the area, highlight him via his/her name and also show their health. Not only is this handy but if you are the first few who spot that player on that players death you gain approx 10EXP as a Spotting bonus, so if you see a swarm running along spam q and spot them all. When they are defeated you can net a good amount of EXP for little to no work.
The other handy thing q can be used for is communicating to other players of your faction without having to open the voice coms to call for Heals, Ammo, Repairs and Pickups. Look at a Medic and pressing q makes you call out for heals, and so forth. Can remove a bit of key pressing in times of need...
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The MAIN Passive method of gain EXP is that of Base Capping. Around the map as you may already know there is plenty of Bases, each varrying in their worth. Large Bases like Labs/Amps/Techs will in fact net you 1000Exp when the base is flipped to your Factions Control. Many other bases will give you 500Exp with the smaller giving you 250Exp. This means that if you focus on taking land you can easily amass Certs due to each 250EXP = 1 Cert...
Another thing to be of note is that taking objectives such as the points and outposts will net you also 100Exp each time it fully flips from theirs to yours. That is a Full Bar of their Faction control to become your faction control. There also be a Cooldown of sorts meaning that you can't abuse this in anyway by having a friend on another faction cap and then you cap back. Untested so not sure. Along with the points are also Generators, in each of the major Bases and a few Mid Level Bases there are Shields corresponding to different grades of either Armour or Infantry, that is if you see a Barrier with an image of a Tank on it, you can't drive through it but you CAN walk though it. Inside each base there is though Generators that correspond to their Shield, Infantry can invade the base and Overload the Generator, doing this will also give you 100Exp...
One of the ways an Infiltrator can make EXP without killing is via Hacking any of the Terminals inside a base, by hacking the terminal you can switch the control of them to your faction for your team mates to use to resupply or spawn armour. As long as you don't have other players trying to destory them for the Exp this can be both a vantage to you in Exp gains and your faction for the ability of change it can present...
There may be more ways to gain Exp, but its best for you to find out yourself and work out how to make yourself useful on the battlefields...
Cya out there...