A small addition for the mopping up of the 'Jack Of All Skills' achievement on the RDA side, especially for the Air Attack/Tactical Attack (or whatever its called in English, since the language of my game is Dutch).
Its best to travel between Tantalus (the very last level of the RDA side) and Hanging Gardens. Because you need to aim at an enemy for the Air Attack skill to work. In Tantalus, where you get off the chopper, you will find a plant (which is considered an enemy on RDA side) right ahead to the left and in the Hanging Gardens you are facing a plant. If you travel to other area's, you'll have to walk further before you can pull off the air attack skill.
Worked pretty well for me. (but the achievement still sucks