Hardly a scientific test, but I had two units of Tercio Pikemen attack (head on, not flanking) an enemy king in a castle square (50 General's Bodyguard). They were in 'spearwall' formation but they were also on the attack and moving, and the bodyguard wasn't moving.

The general's bodyguard got torn to shreds and I lost about 5 pikemen. I've never seen a high-hp unit go down so quickly.