Although the following method most likely works for any type of loadout and skill level, I recommend using four fully upgraded M4A1 assault rifles as soon as you can afford them. Until you have them, simply use this method on normal difficulty and switch to Veteran Difficulty for higher XP gain once you obtain the rifles and upgrades.

Right after you finish the Boot Camp missions go to the main menu.
Select New Mission > Create Mission and choose Type: Hold Position and Map: African Town I

Note: If you can't select a map yet, finish the first mission of the 'Operation Tempest' campaign ('Leave No One Behind')

Once you're loaded into the map, set your Operators up near the door.








Hit End Turn twice and the enemies will start to assault your position.

The first wave will consist of two mercenaries from the left.

Your team will interrupt them and, with a little luck, kill them before they get any shots off.







On Veteran Difficulty, more often than not, one enemy will survive the first round and shoot one of your Operators once.

Use one AP per Oparator to kill him and leave enough AP for interruptions during the next turn.







The next turn will send two more enemies towards you, this time from inside the building.

These should die immediately to the interruptions. If not, again use one AP per Operator to finish them off.






On the final turn, two enemies will rush from the right side and one last enemy will come from inside the building.

As before, interruptions sould take care of every single one of them.





And done. An easy 170 XP and 300 credits (85 XP and 300 credits on normal difficulty)