If you are having threat issues, pop a power adrenal when you first jump in, and start off the fight using a STR stim. Make sure your rotation has hilt strike, guardian slash, and Master strike early. Order the dps on your side to use their aggro dumps.
If taunts and the extra damage from the power adrenal an str stim arent enough, then you might want to double check your itemization, spec, and rotation.
I tank SC on HM at the beginning of the fight as a guardian and have no issues.
when do you use your single taunt? also do you follow it up with the aoe taunt ?