Using this method you can get an Ã¼berteam very early in the game so don't do it if you want to play fair against the alien invaders.
It's very simple really:
1. Go on a mission with some Chrysalises (the more the merrier).
2. Do not shoot the Chrysalises, instead avoid them and kill every other mutant in the area (so you don't get surprised during the leveling process
3. When the area is cleared except for the Chrysalises, lure them to one area of the map.
4. Rush all your men EXCEPT ONE right up to the Chrysalises and watch them get paralyzed (this generates XP like nothing else). Oh yeah, don't forget to turn off the auto-pause for critical hits.
5. Go have dinner, watch a movie or do something useful for as long as you want. The Chrysalises will go on forever shooting those XP-generating rays.
6. Kill the Chrysalises using the soldier you didn't send in to get paralyzed (if you had sent in all soldiers in the beginning they would all have been paralyzed forever...).
7. End the mission and start the leveling up.
Using this method I got over 159000 XP on one mission (6 soldiers, 2 Chrysalises, left it for two hours). My worst soldier was level 4 before the mission. Afterwards he was lvl 18.