There are only two instances of Guardian Angel. The first is a mission in Act I you get from Pierce, the second is in Act II and will be a mission you get from Kinzie (her third, I believe).
Both are missions, not activities, so not missable, but if you want to get this quickly do Kinzie's missions first in Act II.
As far as the activity goes, it's pretty simple. In the first part, you will be sitting on the side of a helicopter following your homie. Your job is to blow up the gang cars that are chasing him/her. Be careful, as your rockets can damage the car you're protecting, so don't fire unless you have a clear shot. The only times I failed were from damaging my own car. The other half is just sniping, and is also fairly easy.
Pierce is rockets then sniping; Kinzie is sniping then rockets.