For boss fights (which means single target enmity), you dont need to use Flash, spamming a 1-2-3 Halone combo should be more than enough. You may weave Circle of Scorn and Spirit Within to increase your dps (and your enmity) during the first combo.
Flash is nice when you have multiple targets, it is not that great for a single mob.

Your Ilvl is more than decent, you should not have any problem holding hate unless your dps start the fight before you even shield lob.