I did it, and even scored a direct hit with the rocket launcher. I think I now know why people tend to hit the ground, and to the side of him slightly. The RPG creates a lot of smoke, which obscures your view, if you you hit the boss directly.

This was enough to daze me for a second, while I forgot which short cut my magnum was on. I wasn't able to get the weapon and aim in time to get a head shot, so after the first body shot I was hit and almost killed (I was doing it on the tiniest amount of health)

So I just start to unload Magnum rounds into it, I think I must have shot it with at least 3, maybe 4 before it went down. I had a slither of health left as well.

For my first time fighting it, I did OK though.

another method (with or without RPG) is to do the fight and then reload your save. Then you just chose to evade him, and save all your ammo. I know you can save before buying the RPG, and then get the kill, reload again and do the fight again (evading) and it saves you the Ptas.

Game plan is to drop 6 x mines on the corner, let him set those off, hit him with a couple of Magnum rounds (which will cause him to go up into the vents) proceed to the next area, dodge the two scripted attacks, and then blast him with the RPG when he pops up in front of you again.

If the RPG doesn't kill Him, another 1 or 2 Magnum rounds should finish the job. You can whiff on the RPG, either more of an indirect hit, or completely miss Him. If you Miss, start again.