Sacred Citadel

Now, this is simple if you have another person with you, but less so if you're doing it alone like me. This is how to unlock it either way, and it doesn't matter if you do it split screen or over Xbox Live. As long as you have at least two people, you're golden. (See below if you don't have a second person.)

Anyway, if you're getting this achievement with a friend, one of you has to be at least level 4. It doesn't matter which class you pick, because all classes unlock the Uppercut ability at level four. For all classes, this ability is , though you can hit up to three times before hitting . After one of you hits the guy into the air, just have the other person hit them with a standard attack before they hit the ground. The attack also works but is harder to time because of the wind-up. Jump into the air if you feel like it. A third or fourth person could also hit the enemy, unlocking the achievement for whoever hit the enemy into/while in the air. You'll all nab the achievement, and then you can high-five each other.

If you're like me and don't have any friends who are willing to come over to help you with achievements, then there's another way. This way obviously works for doing it with two players, but the set-up is more important than above. Plug in two controllers and sign-in. Choose the Mage for one account and whichever second class for the second controller. Level the Mage up to level six. Mage also unlocks Uppercut at level four, but at level six they unlock the Stasis Blast, which is . This is similar to the Uppercut ability, so you can hit up to three times before continuing the combo.

Now, the reason you need to use Stasis Blast is because after knocking them into the air, it will freeze them there for a couple seconds. (Make sure to release before hitting the second time.) This gives you enough time to grab the second controller, run over to the enemy, jump up, and then hit them for the achievement. Since it was just you, you'll have to high-five yourself. It lacks the same "oomph" as high-fiving someone else, but do what you gotta do. And if you're with other people and you're having trouble with the timing using the standard Uppercut, then this Stasis Blast method will work too.

Hope that helps, and good hunting.

Edit: You knock smaller enemies higher into the air than larger enemies. Go for a miner (Act 1, Level 2) or one of the forest beasts (Act 1, Level 3) rather than an orc or brute. The orcs stay in the air for such a short period of time you might not even be able to hit fast enough the second time to actually freeze them in the air. It goes without saying, but huge enemies like bosses or yetis are too heavy to knock into the air.

It's possible to knock the small flying bugs into the air, though it's difficult to do so because they bob and weave so much. Besides that, they don't start appearing until Act 3, and there's no point waiting that long. Piranha-frogs are also a possibility, but they don't appear until Act 2, and again, there's no point waiting that long. If you find you're killing enemies too quickly, switch to weaker weapons at the blacksmith.