i found that the easiest and fastest way is doing mission 4 (the one where you fight jugger and a couple of airplanes) of the hard campaign over and over again.
since it's hard campaign it gives up to 600 exp per co for a total of 1200 exp and with a little practice, that takes about a minute and 2-3 turns.
as a general strategy, the priorities are bomber > b-copter > anything else. just try to kill as much as possible on your first turn, shoot down the bomber with the northern missile, move the southern one forward as much as possible and if you have tag power by your second turn use the first turn to position the missiles to shoot down anything the anti airs can't reach. if you don't have tag, keep at least one anti air in reach of each of your units, so when he gets a second turn and uses it to attack, you can kill anything he has left by your turn 3.
i've heard that mission 20 is supposed to be super efficient for training and while you can use up to 4 commanders and all the oozium will give decent bonus xp, unless you can effortlessly beat it in under three minutes, mission 4 should be by far the most time efficient.
of course, you'll need 2 slots for this, since the xp doesn't carry over unless you save your game and you need to keep the mission 4 save so you don't have to start the entire hard campaign over again.