The end of level 13 offers a decent opportunity for some relatively easy experience grinding.
During the last checkpoint, where Dachev says that they're completely surrounded, a long line of enemy ground units will appear. Destroying them will usually yield 180 to 210 experience per group; if you managed to destroy them all, you'll earn you 2000+ experience. After using all your ammo, just crash yourself or reload the checkpoint and repeat the process. This is the fastest way to grind for exp though you can't use this mission to master other planes since it doesn't have a custom setting option when replaying it. Thus, only the story mission loadout of the SU-27 can be used. You may want to try this trick using Recruit (Easy)difficulty so you can have more ammo. Watch the video
More details can be found in our Guide to HAWX 2