If your doing Mara tanking low level you'll run into things like bad dps/unaware dps an if they hit before you you'll pull agro seen as heavy doesn't have enmity. Also low level Mara can have tp problems.

It gets much much easier when you get to level 30 and get defiance. Since getting defiance I haven't had any class have more than half the emnity bar full. That's just from the standard combo and a couple of overpowers if your on the ps4 it's really easy after your second combo on the main(marked hopefully) mob you can press left and right to do emnity moves on the rest and you won't have to worry about emnity gen or tp problems.

Wait until 30 then it's easy going