Faeria

The only reliable way I Found to trigger this one was to play a functionally mono blue deck and then just add the Swarming Carassius and find time to shoot out a forest and a desert.

Aurora's creation is going to be the mechanism we use to make all this work, and since it costs triple blue, that's why we're going that way. They are only to be used on the Carassius, nothing else.

Again, as with other deck tricks, I believe this can't be triggered in practice games, so I performed this on a wild mission.

3 x Falcon Dive
2 x Plague Bearer
3 x Spellwhirl
3 x Message In A Bottle
3 x Aurora's Disciple
3 x Humbling Vision
3 x Water Elemental
3 x Frogify
3 x Aurora's Creation
3 x Swimming Carassius
1 x Compass of Gagana

Basically, Aurora's Disciple is going to do a lot of the grunt work, and you want to have a turn where you rattle off a Carassius & a Creation on it on the same turn. As you play them, have them all die before you play the next one. Plague bearer is in the deck to do a big sweep if you need to clear them out if the computer hasn't cast a fattie or two to suicide them into.

Play one, one dies.
Play the second one, three are dead.
Play the third one, seven are now dead.
Play the fourth one, fifteen are now dead.

Then you have two others in your hand from drawing the 3 naturals or the aurora creationed ones, make sure you have enough board space, and summon the horde. You'll need to complete the game for the achievement to trigger, but once you've cast 20 3/2 flyers on one turn, that's usually not an issue.