The second tier of Aerial Rave requires you to perform 25 air combos of at least 3 hits.
For this achievement, you'll have to use a launcher to knock your opponent up into the air, and then follow up with a chain combo with at least 2 more hits in the air.
Launchers are moves that knock your opponent up into the air. In MVC the universal input for launchers is +. In MSH, the button for launchers is different for each character. Some characters may also have more than one launcher. The launcher appears to count as part of the air combo, so after doing a launcher you only need to get 2 more hits in the air to get it to count towards your total number of aerial raves.