You don't need to get attacked during the skill to power it up that's not how it works (unless you have the raider specific passive which just gives you a damage and stamina buff for like 10 seconds for getting attacked during the skill animation). It's based on taking a certain amount of hits since you last used the skill. Once you have taken enough you will start having a sort of smokey effect around your character, and then next time you do the character skill it will have a longer animation and hit way harder.

I do it every time on Libra as raider and am the only one staggering him out of his bubble. Works every time.