At low levels, you're doing pretty much exactly what you're supposed to do. At 22 you get provoke, so if you lose something, you can voke/shield lob it and that's usually enough to hold it long enough to drop another flash.
Don't be afraid to stop your combo on an enemy to drop another flash. Be sure to cycle targets just to check aggro meters. Flash is your go-to aoe aggro pull until you hit 50 where you'll get another one. I wouldn't use Riot Blade because there's no enmity generation to it. Once you have a decent bit of aggro on the groups, you can drop Riot Blades to regen MP if it's pretty low. Otherwise, natural MP regen should be enough to get you from pull to pull.
Finally, yeah, what tigerkiddo said, once you feel comfortable, start dropping savage blades on different targets whose aggro are relatively high to drop it back down. Once you hit 26 you'll get Rage of Halone, and you'll be using Rage of Halone until... expansion? Probably past expansion.