Honestly if you do it yourself you will feel so much better. I landed this mission first time, but it was more luck than skill.
I would recommend you use your map as much as possible to see where your destinations are and to see where you think Mendez will be going. I believe he follows the same route every time. I assume you are failing on the last bit when you have to pick up Mendez? If you are then I would recommend you try and guess where he is going to go and be there before him. I found he weaved a lot, so I hammered it in a straight line down the road he was on and eventually managed to latch on to him. Just remember when you do get him to climb quickly and don't let his car hit anything. The damage indicator will not go and you can still trash his car.
Trust me, you can do this mission, and throwing your PSP against a wall will not do it any good. You can ask my PS2 controller about that ever since I came across the RC missions in San Andreas. May it rest in peace
Good luck and keep trying