Gaston Phillips is a pain in the ###.

[Mike Hawthorne]

I spent an hour trying to wear him down, pretty sure I had him, got him down to 14% damage and then at the same time that I'm pouring shots into him I see his health go back up to 68%. This is right when I think I'm finally going to finish him off.
I gave up, I'll try him again when I'm tougher.

If the hadn't suddenly restored most of his health I would have won.

[Bogmore]

Get onto the roof next to the street and shoot him in the head with a sniper rifle a few times.

[Mike Hawthorne]

I'll give it a try.

[Grobut]

I just snuck up on him and chocked him out.

[Tokenn]

This post would be more informative if you told us your Overall level and Street Cred level. Some of the 'boss' enemies do regenerate their heath, and it can be pretty frustrating...