Campaign takes quite a long time and you run out of new stuff to try relatively quickly. This means that quite often you will just repeat same strategy that has proven to be the best. It is quite useful to level your officers early. You can optimize xp gain by making officers do most of the work. To gain new officer you need to ensure a basic soldier of a specialization you want (Engineer, Marksman, Vehicle crew, etc.) will gain enough xp to level up in a mission. You can only choose one basic soldier to promote to officer after a mission so no need to try to level multiple soldiers at once.

1. Operation Swift Phoenix - Somalia
   
2. Operation Garden Light
   
3. Operation Distant Sweep
   
4. Operation Crimson Harvest
   
5. Operation Decisive Shield - Bosnia
   
6. Operation Noble Horizon
   
7. Operation Infinite Response
   
8. Operation Eternal Guard
   
9. Operation Intense Harvest - Afghanistan
   
10. Operation Urgent Arms
    
11. Operation Ghost Promise
    
12. Operation Enduring Forge
    
13. Operation Vigilant Arms - Colombia
    
14. Operation Fiery Star
    
15. Operation Green Anvil
    
16. Operation Quick Wind - Iraq
    
17. Operation United Thunder
    
18. Operation Silent Hope
    
19. Operation Golden Obelisk
    
20. Operation Red Sentinel

To unlock abilities you need to level your officers by having them gain xp. You can gain xp by shooting, repairing (Engineer) or healing (Medic). List of officers I found most helpful:

1. Engineer
   
   • can repair own vehicle (but not to full)
     
   • can make FRSVs repair/rearm faster and use less supplies
     
   • very useful in any mission you have access to FRSV
     
   • has LMG
2. Marksman
   
   • can one-shot infantry
     
   • can regenerate health and have high amount of health
     
   • useful for clearing enemies from buildings from distance before you send in vehicles
     
   • useful in any infantry mission
3. Vehicle Commander
   
   • can repair own vehicle (but not to full)
     
   • packs a punch when in a tank
     
   • improves damage of nearby vehicles
4. Pilot
   
   • improves damage and vision range of aircraft
     
   • improves damage of nearby aircraft

• Helicopter - has anti-tank missiles
  
• Fighter Jet - hovers around like a helicopter, has anti-air missiles, bit more resilient than a helicopter