Iron Man 2

The unibeam is done by charging an attack with LT+RT while on the ground or hovering. Light surrounds you and when you hear the sound effect, your charged.
This is easy to do during the GreenGrid mission (when escorting the Arc Armature near end of campaign.) There are a ton of small drones that don't require a full charge, if your upgraded at all even the stronger bots dont pose much of a challenge, and at the very end of the level (the large circular area,) there is an infinite spawn.