Fastest Way To Level Up

I have been playing a lot of missions lately and i got 30k kills now and i've been doing ally missions nightly rewards but im still 23 after 2 months of playing

Here's what you do, unlock some missions, click on the ones that give the most experience. Note: the ones that give a large amount of cash will give less experience. Then, use your energy wisely to click on the mission that takes one day to reward a lot of experience because it's worth the amount of wait time. I generally do this because people who want quick experience get less from doing shorter missions which is why the longer ones give more exp.

I agree. Keep unlocking new missions in the campaign and do the allies missions! ^_^

Just want to make sure that you don't have xp gain disabled by chance. If you see a flashing red exclamation point in the upper left corner of the screen....you are not currently collecting xp.

Like suggested above use ur allies wisely,try to unlock as many missions (and allies) as possible, some allies give more experence than others.
When im playing I tend to use 2 hr jobs, when im not playing before logging off i activate 24 hr jobs.
Also like to point out that certain mission give more experience than others. The missions on the Fresnel map give the most experience (20 exp/mission) so try to do these as much as possible.
Do missions together with a friend, this way u can finish the mission faster than when ur doing it alone. Also u do not have to kill every enemy in a mission, run as fast as u can to the endboss defeat him and leave.
Finally when u level up ur energy is reset to 25, right before u level up spend ur energy, this way u maximize ur energy usage.

Thanks Guys