Assuming this guy didn't get multiple copies of the same game, go to any events for legendaries catchable ingame, engage in loads of outside trading, or simply hack, this would require:
-Ruby (or Emerald)
-FireRed
-LeafGreen
-Colosseum
-Diamond
-Pearl
-Platinum
-HeartGold
-SoulSilver
-And, finally, Black 2, for a total of 10 games.

This is really unfortunate for people like me, who stopped playing Pokemon with the 3rd Gen and came back for X and Y.
Oh well, at least there are som people who don't want Yveltal for every Pokemon on GTS.