Well, there's a reason why Mihile ranks so low on actual DPS charts, it's Royal Guard being finnicky. Most of the time, you will not have multiple stacks of the buff, or even one active. I'd actually recommend Paladin outright over Mihile right now, they do a lot of things better and while Paladin is also very much on the bottom of the DPS chart, they actually live up to their number much more than most classes do and have a attack that can bind. They also don't need to toggle a skill on/off for single targets.
That being said, if you get hit before using Royal Guard don't use it until you stop flashing, that second of invincibility will stop it from working. Avoid any lines of potential/nebulites that increase invincibility duration upon being hit as well, these increase your chances of failing to use Royal Guard effectively.
