You'll start in the middle of Diaz turf (in their motor showroom in fact) and with only a chainsaw to your name. At first it's simple enough; they're all armed with melee weapons and the chainsaw is quite a weapon at close range. Don't forget to swing to slice off arms, legs and heads for extra balls. Collect those packages to get some free coke and after you've taken out everybody upstairs, head down and take out those waiting for you. Now is where it can be annoying. In the next building, they're armed with long range weapons, so enter blind rage mode and take them out that way, then pick up the weapons they drop so you can use them when the feeling takes you. Now head into the manager's office and kill Edgar.
Grab the car and go after the remaining Diaz brother. Shoot at the car and once it starts flaming, Alonzo will get out. Simply shoot him dead.