Yeah, I had the same problem in low level dungeons, but as people say, it gets better as you level. Usually I had to swap between targets, like from an orange enmity to one at green, and then swap back to the other one. This gives tanks some time to build more enmity. To refresh Thunder is another good way to give tanks some time to gain enmity, without losing your DPS. At my current level, most of the times I can spam spells, even hard procs, without pulling aggro, but again, it depends on the tank! I pulled even Garuda now and then back in lv45 playing with bad tanks...