The Heavy Mooks in question are found outside the Horde fortress - between the beach and the walls facing the Skyfire; and inside the fortress in the outer courtyard. They have a big heap of health for a non-elite mob, as their name implies, and their abilities (which are a melee 'smash' and a 'shattering throw') are not interruptible. This isn't much of a worry unless you're barely geared for these quests.
On the upside, you can get help from allied mobs in the area, so you can pick them off with a little help from these if you find them too daunting. Be cautious if you pull them towards the Shieldwall Marines, though. They'll gladly engage, but they do a huge knockback attack which may bring other mobs into the fight.
The SI: 7 Saboteurs are more subtle comrades in arms for these.