Hating the Haters
Head over to the jogging area. Get your automatic weapon ready; you will need it later. Follow Bernie but keep your distance until he reaches the tunnel. After Bernie gets battered, chase his attacker up the stairs. He will flee on a scooter. If you stop to shoot at him, you will not be able to hit him; meanwhile, you will attract the police. Instead, as soon as you get to the street level, steal the scooter to your left and take off after him. He normally takes the second path on the right and goes in front of a building, then turns down a path and goes back the direction you just came.
A simple kill is to stay on the street after stealing the scooter, turn right at the intersection, go down to the building and you should see the target on his scooter. Swerve off the street to follow him but after you get through a tunnel, he will go to where you first started following Bernie, then circle back on the lower path to your left that you jogged down before.
Instead of following him, go down the grass to the left and hop off the scooter. If you didn't already get your automatic weapon ready, get it now. When he comes down the path, lock on and shoot. He will probably get by you, but you can turn, lock on and continue shooting. You should be able to kill him without getting back on the scooter.
The shooting will give you a star or two, so drive to try to get to an edge of the wanted circle where you do not see any cops around. You should be able to lose the wanted level without leaving the park, then go pick up Bernie.
This has been tried repeatedly with success, but if he happens to take a different route, follow the same strategy of trying to cut across the grass ahead of him. He stays in the park a long time, so you should get several chances. If he leaves the park and you come across Bernie wanting to get on the scooter, leave Bernie there and come back for him after getting the thug.
After watching or skipping the cutscene, on the very left of where you are standing is a very fast sports car and is a must have. Get in it, pick up Bernie and follow the yellow marker on the map. After watching the cutscene, chase after two of Dimitri's thugs. Keep up with them and try to avoid running into the flipping cars dashing at you. As soon as Dimitri's thugs crash their car, park up near their wrecked car and take them out as they run away. Sometimes the police will come after you during this. If they are alerted, get back in the car with Bernie, evade the cops and return to Bernie's place.