Best method (in my opinion), is getting a second controller and doing riot shield boosting. in the 70's I leveled up about 4-5 times in 1 game

Anyone got a better idea or the same idea but explain more in detail and say the approx xp you will recieve?

Alright, this is what I do.
-Get 4 friends who have 2 controllers (1 main and 1 guest)
-Play Hardcore Tac (This way you can take your guest and shoot at your own riot shield)
-Give your guest custom loadout an LMG and your main account a riot shield
-Wait 20 seconds after unloading a whole clip from the LMG before shooting again to get the most amount of points you can
-Rinse and repeat

how much xp do you roughly get a game? and is it better than just getting kills in search?

Roughly I'm not too sure mainly because I did it on Hardcore and it doesnt indicate how much XP you're earning, but I did it with Double XP on and I felt as if after every spray (in the lower levels) I leveled up each time.

I suggest demolition kill boosting, no turns and at 70+ you rank about once every 7-10 minutes.