Resident Evil 4

After the cutscene, climb the ladder behind you and go to the left. There are some boards that are blocking the way. Get behind these boards and look down. Mendez, in his first form, will continue walking (looking up at you) towards the wall with no attack. Shoot him until a short cutscene kicks in. Once that is finished you'll be in the second phase of the boss fight. Mendez in his second form is a bit harder, but if you can position Mendez to the left of the boards and stay in the right corner, he will attempt to hit you with his claw, but if you are positioned right he won't hurt you. He'll keep doing it, and be completely open.

Shoot him repeatedly, and the fight will be over shortly. From there, collect the items in the area, the cash Mendez dropped, and most importantly -- the False Eye, because you won't be able to come back once you leave through the hole in the wall.

However, if you don't want to exploit that glitch... keep in mind that Mendez is weak to incendiary grenades. So use them, as well as the explosive barrel to your right when the boss fight begins.

You no need to use Rocket Launcher to kill him, that will make you wastle a lot of money. He has tow forms to defeat. Once can walk slowly, the other is fly on the roof.
He will be very weak with incendiary grenades in the first form. If you are playing Normal or Pro mode, with 3-4 incendiary grenade will defeat him in the first form. In his second form, use TMP to shoot him. When he run near you, run away and shoot with TMP, keep dodging his attack in second form. 150-230 TMP will kill him completely when in second form.
Don't go to the ladder while he is still staying in the first form because he can damage you a lot. When he ided, take the False Eye and the hole on the wall will open. If you jumped out, you will never jump in again so decide carefully the items before leaving.

You can use a knife only on the 2nd form..Watch his swings.( being on 2nd floor).When he 1st grabs the beam slash 2-3 times with your knife..My next move is to jump down.Run to the door, and wait for him to head your way..Climb back up & repeat..This can take awhile, but big fun too.

The easiest way would be saving up about 5-6 incendiary grenades... just throw it at him and wait until he gets back up and repeat. It will kill him very easily because he is weak against fire

The easiest way I've found to beat mendez is to use incendiary and hand grenades until he splits,
then as he swings in the rafters, blast him with the shotgun every time he gets close.
I just plant myself in front of the door and keep fighting. If he swipes at you, use the reflex command to dodge.

In his first form, climb up the lader(it doesn't matter which)Get the hand grenade throw it at him .then keep running back and forth shooting him with any gun.When he splits into two ,keep shooting him with the shotgun.if he gets to close to you run to the other side of the building and do the same.Repeat until dead.