you should prolly be lvl 25 but lvl 20 is the minimum.
load hacker and download mission map plus spy all the way as much as much as you can.
for more exp have a level 50 in your group and he doesent have to do anything at all just be in the group he can even go pvp

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load a infiltration mission small/hard and make sure its not a mission with killing needed if it is abort then reload it and if it keeps comming up the same one then go to mara c and the missions will reload. go to the first destination and put your desguise on and run in there fast do what ya need to do and get out! same with the other destinations you can get