Here's a nerve-bar for me (I guess you can call it that)
When you reach Satellite Scan - meh. Ok.
When you reach Airstrike - Hmm. not bad. Still not intense.
When you reach the Turret - Pretty good kill chain I got here. Fairly intense.
Helicopter Support - Intense.
Bomber - Intense as hell.
15-18 kills - Your heart is racing like nothing else.
19 kills - You start becoming hot and you can feel your heart on your chest.
20 kills - HELL YEAH!