The trigens are difficult to kill, but not impossible. I found killing them was much easier when using a shotgun rather than an assault rifle of some type. Use assault rifles to kill the mercenaries and save the shotgun ammo for the trigens. It helped a lot for me. Always reloaded all your weapons after each battle too, no matter how small. In some of the larger battles you'll need to switch guns quickly because reloading during a battle takes too long.
But, in the end, there's no getting around the fact that FarCry is bloody hard near the end of the game.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.0.11) Gecko/2009060215 Firefox/3.0.11 (.NET CLR 3.5.30729)
Core i7 920 | ASUS P6T Deluxe v2 | 3TB+ HDD | 12GB Corsair DDR3 | Radeon 4890 Xfire | X-Fi Titanium Fatal1ty | Logitech Z-5500 Speakers | Dell 3008WFP | Seven RC1