Shoulder charge is like Fist of Havoc or slide-shotty. The Guardian has to commit to attacking a spot. You counter it by simply not being there. Especially since you can't stand your ground and fight with any of the game's primary weapons. The easiest way to evade is to give ground and get vertical....and if you can stick a fusion grenade tot he head of the Titan while you're at it. So much the better.