Well, first you need an Over Boost unit that supports Assault Armor. Then, just hit both of your weapon switch buttons and the OB button at the same time.
Here's a list of the OB units that can use AA.
- GAP-AO/CG
- I-RIGEL/AO
- KRB-PALLAS
- KRB-JUDITH
- KRB-LAHIRE
- KRB-SOBRERO
- ARGYROS/AO
I think I might have messed up on one of the names, but that looks right to me. Edit: Just a reminder, though. AA enabled OB units sacrifice most of their OB power in return for their AA capability. Regular OB units allow greater mobility, but AA units offer versatility.