First, get a nice, fast car in a good condition or you could take his car if you really wish. Either way, get to the North Beach bank. Walk in with your weapon drawn – this way you'll be surprising them instead of the other way around. Take out all of them in the bank and be careful, as they'll be all over the place. Don't forget to check behind you. After you've gotten rid of them, some will run off. Get after the stragglers, ignoring the cops. Keep a little distance to prevent your car from being full of holes, and follow him until he reaches the gang's hideout at the top of a multi-story car park. This can be quite hard – if you've got blind rage, then use it and clear out the gang in seconds. Otherwise, if you go up that ramp you'll be entering a wall of bullets. Either take a good armoured car up there or wait for some of them to come down to meet you. You don't have to kill them all. While you're up here, you might as well drive off the roof using that ramp if you want to.
You can now buy the store for $12,000,000. If you have Montana Holdings it will cost you $10,800,000.