This is a Brodie Gang mission.
"The time has come to pay Tam a visit. You have terrorized him, wiped out his gang, destroyed his livelihood. Now all that remains is to kill the stupid fuck. He should have known better than to have crossed you..."
Well, it's finally time to put an end to Tam and wipe out his gang for good. Tam has taken refuge at the Empire Bay Forge with the remnants of his gang for one last stand against Jimmy. If you've played the main game, this is basically the same as the battle against the Greasers in The Wild Ones.
You'll want to stock up on ammo and health, but all in all this one isn't too bad and you should get through it without much trouble. Activate the mission and drive to the foundry. All together you'll have about 30 thugs to kill here, so run in and engage the first group. Just work your way along taking them out as you go. As you approach the first junction another large group will come from around the corner. Pick them off and then turn right.
If you look you should be able to see Tam running scared further into the back. He's the one with the large green arrow above him. If you're lucky you might be able to take him out now, but what fun would that be? It's best to save him for last. Get some cover and pick these guys off as you head further back. When you reach the end of this second leg Tam may be standing right there making an easy target of himself; if not or you miss he will run further along to the end of this next area.
As you continue through here be mindful of the guys on the catwalks above. They can be tricky little buggers and often stick in their cover, waiting until you pass them to open fire. If you see a red mark on your mini-map but can't find the guy, try throwing a grenade or molotov to any area above where he might be hiding.
If you get to the end of this leg and still haven't taken out Tam, he will run out of the forge and head down the road on foot. Time to run after him and end this once and for all. Once he's dead, head to the safe zone just outside the entrance to the forge and end the mission.